A mathematical way to think about biology
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A mathematical way to think about biology comes to life in this lavishly illustrated video book. After completing these videos, students will be better prepared to collaborate in physical sciencesbiology research. These lessons demonstrate a physical sciences perspective: training intuition by deriving equations from graphical illustrations.
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Lecture 1  01:48  
This video outlines contents from the course 

Section 1: Using deterministic models to study aspects of stochastic systems  

Lecture 2  05:15  
Concepts of stochasticity underlie many of the models of dynamic systems explored in quantitative biology. We describe some of these ideas in this and the following three videos. In this video, we state that systems exhibiting deterministic dynamics can sample a messy variety of waiting times between chemical reaction events even when the motions of component parts are periodic. Particularly, this can happen when the periods of motion of individual parts are incommensurate (pairs of periods form ratios that are irrational). 

Lecture 3  04:29  
In a deterministic system with complicated interactions, small differences in initial conditions can quickly avalanche into qualitative differences in dynamics. Since initial conditions can only be measured with finite certainty, the dynamics of such systems are, for practical purposes, unpredictable after short times. 

Lecture 4  05:03  
In the previous two videos, deterministic systems displayed dynamics with aspects associated with stochasticity. In contrast, some systems not only mimic some aspects associated with stochasticity, but, instead, display indeterminism at a fundamental level. For example, when a collection of completely identical systems later displays heterogeneous outcomes, the systems are fundamentally indeterministic. They have no initial properties that can be used to discern which individual system will display which particular outcome. 

Lecture 5  04:46  
Markov models are often used when developing mathematical models of systems which partially or more fully display aspects associated with stochasticity (depending on how fully a system displays aspects associated with stochasticity, the use of a Markov model might need to be recognized as a conceptual approximation). Icons that can represent the use of such models include spinning wheels of fortune and rolling dice. 

Lecture 6  08:57  
In this and the following three videos, we present a canonical worked problem that is presented in introductory systems biology coursework. For an example of this mathematical lesson, see Alon, Ch. 2.4, pp. 1821. In this video, we animate a time sequence of translation and degradation events that cause the number of copies of a protein of interest in a cell to change over time. 

Lecture 7  09:50  
We derive a differential equation approximating the timerate of change of the number of copies of protein in the cell modeled by the animation in the previous video. This differential equation reads, dx/dt = β  αx. We depict aspects of this differential equation with a flowchart. It is important to remember that this differential equation does not represent all aspects of the stochastic dynamics in the toy model presented in the previous video. 

Lecture 8  04:12  
We sketch a slope field corresponding to the differential equation derived in the previous video. We use this slope field to draw a qualitative curve describing how the number of copies of protein is expected to rise over time, when starting from an initial value of zero. 

Lecture 9  11:02  
We obtain an analytic solution for the relationship between the number of copies of protein and time for the differential equation qualitatively investigated in the previous video. We find that the rise time, T1/2, is ln(2) divided by the degradation rate coefficient, α. The fact that the rise time is independent of the translation rate β is sometimes used as a pedagogical example of the importance of quantitative reasoning for gaining insights into biological dynamics that would be difficult to develop through naturallanguage and vaguelystructured notional reasoning alone. 

Lecture 10  13:02  
Using a collision picture to understand why reaction rates look like polynomials of reactant concentrations 

Lecture 11  11:58  
Cooperativity of a simple (oversimplified) kind 

Lecture 12  06:32  
How Hill functions, considered in combination with linear degradation, can support bistability 

Lecture 13  15:35  
This video introduces collisional population dynamics and tabular game theory (comparative statics). The particular game in this example is the prisoner's dilemma. In this game survival of the relatively most fit occurs simultaneously with decrease in overall fitness. For a printable tutorial explaining how evolutionary game theoretic differential equations can be applied to analyze population dynamics, please refer to doi:10.1098/rsfs.2014.0037. 

Lecture 14  12:32  


Lecture 15  19:20  
In the previous slide deck, we noted similarities between population dynamics and business transaction payoff pictures. In this and the next video, we provide deeper understanding of these connections. In this video, we derive the population dynamics equations in such a way that it is natural to say that cells being modeled repeatedly play games and are subject to game outcomes. For a printable tutorial describing interpretations that can be associated with evolutionary game theoretic differential equations, please see doi:10.1098/rsfs.2014.0038. 

Lecture 16  10:58  
Repeated simple interactions in a population of robotic replicators can achieve results seemingly related to results obtained from sophisticated computations. The use of population dynamics and business transaction payoff matrix analyses from the previous slide deck to obtain this understanding is an example of quantitative reasoning.  
Section 2: Probability and statistics  
Lecture 17  05:54  
The first of five videos on introductory statistics, this module introduces probability distributions and averages. The average (also called "arithmetic mean") quantitatively expresses the notion of a central tendency among the results of an experiment. 

Lecture 18  03:43  
The average of a sum is the sum of the averages. The average of a constant multiplied against a function is the constant multiplied by the average of the function. The average of a constant is the constant itself. 

Lecture 19  05:33  
The variance of a function is the average of the square of the function. For the purposes of theoretic calculations, it might be useful to express the variance using the "insideout" computation formula described in this video. 

Lecture 20  06:26  
Two variables are said to be statistically independent if the outcome of an experiment tracked by one variable does not affect the relative likelihoods of different outcomes of the experiment tracked by the other variable. The twovariable probability distribution factorizes into two probability distribution functions. 

Lecture 21  07:20  
The covariance of statistically independent variables is zero. The variance of a sum of statistically independent variables equals the sum of the variances of the variables. This identity is often used to derive uncertainty propagation formulas. 

Lecture 22  03:49  
This slide deck provides examples of how hypotheses about probabilistic processes can be used to discuss probability distributions and obtain theoretical values for averages and variances. In this first video, we describe the Bernoulli trial, which corresponds to the experiment in which a coin is flipped to determine on which of two sides it lands. 

Lecture 23  07:18  
In this second video in this slide deck, we discuss the binomial distribution. This distribution describes the probability of getting x heads out of N coin tosses (Bernoulli trials), each individually having probability p of success. 

Lecture 24  08:13  
In the Poisson limit, we take a series of [independent] Bernoulli trials (giving rise to a binomial distribution) and allow the number of coin flips N to increase without bound while allowing the chance p of success on a particular coin flip to decrease without bound in such a compensatory fashion that the average number of successes ("heads") is unchanged. Because the likelihood of "heads" on any given toss decreases without bound, this limit is called the limit of rare events. 

Lecture 25  11:22  
To study the combinatorics involved in an example where the central limit theorem applies, we will need to work with the factorials of large numbers. Stirling's approximation is an approximation for n! for large n. In this video, we motivate this approximation by comparing the expression for ln(n!) with an integral of the natural log function. 

Lecture 26  06:37  
The central limit theorem states that a Gaussian probability distribution arises when describing an overall variable that is a sum of a large number of independently randomly fluctuating variables, no small number of which dominate the fluctuations of the overall variable. 

Lecture 27  09:01  
In some situations, when the number of coin tosses is large, Stirling's approximation can be applied to factorials that appear in the expression for the binomial distribution. The resulting expression is basically an exponential function of a quadratic function with a negative leading coefficient. This is the hallmark of a Gaussian distribution. 

Lecture 28  03:50  
For a Gaussian distribution, roughly twothirds of the probability is found within the first standard deviation. 

Lecture 29  08:25  
Because equipment in physics experiments is highlyengineered, individual device contributions to measurement fluctuations might be "small." The overall fluctuations in the final measured quantity might be well approximated using a firstorder Taylor expansion in terms of individual device fluctuations. Fluctuations in measurements are thus sums over random variables, and thus, potentially Gaussian distributed. 

Lecture 30  10:35  
The levels of molecules in biological systems can approximate "temporary" steadystate values that equal products of rate coefficients and reactant concentrations. Since logarithms convert products into sums, the logarithms of the levels of some biological molecules can be normally distributed. Hence, the levels of the biological molecules are lognormally distributed. 

Section 3: Uncertainty propagation  
Lecture 31  07:45  
Quadrature formula is a result of Taylor expanding functions of multiple fluctuating variables, assuming that fluctuations are independent, and then applying the identity "variances of sums are sums of variances"  
Lecture 32  09:38  
Standard deviation vs. sample standard deviation Mean vs. sample mean Standard deviation of the mean vs. standard error of the mean 

Lecture 33  05:57  
Origin of the famous factor of sqrt(n), which is the ratio by which the standard deviation of the distribution of the sample means is smaller than the standard deviation of the distribution of the measurements (parent distribution)  
Lecture 34  03:18  
Are error bars nonoverlapping, barely touching, or tightly overlapping? What pvalue do people associate with the situation in which error bars barely touch?  
Lecture 35  06:55  
"I quantitated staining intensity for 1 million cells from 5 patients, everything I measure is statistically significant!" It is quite possible that you need to use n = 5, instead of 5 million, for the √ n factor in the standard error. 

Lecture 36  08:18  
In order to identify theoretical curves that closely imitate a set of experimental data, it is necessary to be able to quantify to what extent a set of data and a curve look similar. To address this need, we present the definition of the quantity chisquared. For a given number of measurements, a smaller chisquared indicates a closer match between the data and the curve of interest. In other words, a smaller chisquared corresponds to a situation in which it looks more as though the data "came from" Gaussian distributions centered on the curve. The average chisquared value across a number of experiments, each involving M measurements, is M. 

Lecture 37  13:37  
We slightly modify the definition of chisquared developed in the previous video for the situation in which a "correct" curve has not been theoretically determined beforehand. We choose a "best guess" curve with corresponding best guess values of fitting parameters by minimizing chisquared, which corresponds to maximizing likelihood. 

Lecture 38  03:44  
Using the concepts developed in the preceding two videos, we present a checklist of steps necessary for performing fitting of mathematical curves to data with error bars. These steps include checking whether the reduced chisquared value is in the neighborhood of unity and inspecting a plot of normalized residuals to check for systematic patterns. This algorithm is appropriate for general education undergraduate "teaching laboratory" courses. 

Lecture 39  5 pages  
The purposes of this exercise are (1) to practice sample variance curve fitting in MatLab and (2) to understand that the timesequence according to which data are acquired can affect the apparent size of error bars. This exercise is a PDF, instead of a video, so that you can refer to a printout next to your computer as you work through the commands described.  
Section 4: Computation of stochastic dynamics  
Lecture 40  07:49  
Dynamics of population probability distributions can be described by using differential equations. 

Lecture 41  03:13  
In this and the following two videos, we present the stochastic simulation algorithm. To apply this algorithm, we need to specify the kinds of reactions that a system can undergo, we need to determine waiting times that elapse between consecutive reactions, and we need to determine the identities of the reactions that occur. In this first video, we illustrate how a systems' possible reactions are specified by specifying reaction rates and stoichiometries. 

Lecture 42  10:45  
Use a pseudorandom number to specify a duration of time, drawn from an exponential distribution, that elapses until the next reaction. 

Lecture 43  03:18  
Use a pseudorandom number to choose, with probability proportional to average probability rate (propensity), a particular reaction type for the next chemical reaction. 

Lecture 44  08:11  
Model: RNA polymerase makes many (usually unsuccessful) independent attempts to initiate transcription and mRNA strands degrade after a precise lifetime Outcome: mRNA copy numbers are Poisson distributed 

Lecture 45  14:11  
Model: RNA polymerase makes many (usually unsuccessful) independent attempts to initiate transcription. Once a mRNA strand is produced, it begins to make independent (usually many unsuccessful) attempts to be degraded. Outcome: As in part a, mRNA copy numbers are Poisson distributed 

Section 5: Linear algebra  
Lecture 46  05:19  
Motivating example: Modeling dynamics of web startup company customer base  
Lecture 47  09:56  
Vectors, vector spaces, and coordinate systems  
Lecture 48  13:44  
Linear operators, matrix representation, matrix multiplication  
Lecture 49  09:35  
Using eigenvalueeigenvector analysis to solve for the dynamics of the demographics of the webstartup customer base. First, we qualitatively describe the longterm behavior of the system in paying/nonpaying customer population space. 

Lecture 50  17:50  
Now that we have qualitatively described the longterm behavior of the paying and nonpaying customer populations in this model, we obtain mathematical descriptions using eigenvectoreigenvalue analysis. 

Lecture 51  06:37  
Simple quasispecies eigendemographics and eigenrates based on Bull, Meyers, and Lachmann, "Quasispecies made simple," PLoS Comp Biol, 1(6):e61 (2005) In this first video, we obtain discretetimestep population dynamics equations by considering proliferation and mutation events at the level of the single cell. 

Lecture 52  06:42  
We use eigenvalueeigenvector analysis to describe the longterm steadystate population composition. We find that relative dominance in a population is determined, not merely by "fitness" alone, but also depends on the degree to which individuals "breed true." 

Lecture 53  04:29  
Euler's formula: Expanding the exponential function in terms of sine and cosine Complex exponentials in the complex plane Euler's identity exp(iπ) = 1 

Lecture 54  04:51  
In this and the next video, we develop a familiarity with the representation of vector rotations using rotation matrices. This understanding is helpful for identify dynamical systems that support oscillations in physics, engineering, and biology. A rotation operator rotates a vector by an angle without changing the length of the vector. A rotation matrix represents the action of a rotation operator on a vector. 

Lecture 55  08:43  
How can we determine whether a dynamical system can be represented using something that looks like a rotation matrix? Rotation matrices have complex eigenvalues. We can determine whether a dynamical system supports rotational motion by determining whether the matrix representing the system's dynamics has complex eigenvalues. 

Section 6: Differential equations  
Lecture 56  11:03  
CAUTION: I'm not familiar enough with numerical integration to know whether the particular example of the method for stepsize adaptation in the video is used generally (or at all) in commonly available software packages. The purpose of the example was to show that it is possible to generate an error estimate (a) without knowledge of the actual solution and (b) by comparing the solutions from two numerical integration algorithms. 

Lecture 57  03:52  
In this and the following three videos, we present a canonical introduction to mRNAprotein system from systems biology 101. In the fourth video in this slide deck, we summarize the process of linear stability analysis that can be applied to systems of differential equations that can be expressed in the form of 2x2 matrix equations. In this first video, we obtain the system of differential equations describing this model by presenting assumptions that mRNA molecules are transcribed and degraded and that copies of protein are translated and degraded. 

Lecture 58  07:17  
We identify particularly simple, onedimensional, trajectories of the transcriptiontranslation model through mRNA levelprotein level state space. 

Lecture 59  16:30  
Some of the trajectories in mRNAprotein level state space are onedimensional (unbending). This insight allows us to learn that the dynamics of the vector in mRNAprotein state space are described by a linear combination of eigenvectors with weighting coefficients that are exponential functions of time with coefficients equal to the corresponding eigenvalues. 

Lecture 60  11:49  
Eigenvalues and eigenvectors of a linear system can be used to classify a critical point as a source node, sink node, saddle, source star, sink star, source degenerate node, sink degenerate node, source spiral, sink, spiral, or center. 

Lecture 61  07:49  
Adaptation is not absence of change; instead it is the presence of eventually compensatory changes See also: Read Ma, Trusina, ElSamad, Lim, and Tang, "Defining network topologies that can achieve biochemical adaptation," Cell 138: 760773 (2009). In this video, we describe an example of an incoherent feedforward loop molecular circuit topology, which, as we learn in the following two videos, supports adaptation. In the fourth video in this slide deck, we summarize the method of almost linear stability analysis that can be used to study systems in which the differential equations cannot be expressed in the form of a matrix equation with constant coefficients. 

Lecture 62  06:13  
Adaptation is the eventual restoration of the level despite the lasting presence of a change in a stimulus that temporarily caused a change in the read out. The incoherent feedforward loop is one way to use three nodes to produce this effect. After the level of input A rises, activation of read out C rises, but inhibition of C through B also rises. The final steadystate level of read out C is unchanged. However, since the level of inhibitor B takes some time to rise, inhibition of C is temporarily insufficient to compensate for increased activation of C by A. Thus, the level of C is temporarily higher before it approaches its original value. We visualize nullclines and critical points in the BC phase portrait before and after a step change in A. 

Lecture 63  11:06  
The system of differential equations describing the incoherent feedforward loop in this example cannot be directly expressed in the form of a 2x2 matrix equation with constant coefficients. A power series expansion is used to identify higherorder terms that are neglected in the vicinity of the critical point. The remaining portion of the system of differential equations is linear and can be analyzed using eigenvalueeigenvector methods. The dynamics obtained are consistent with the dynamics described more qualitatively in the previous video. 

Lecture 64  05:48  
Even though an almost linear system is not exactly a linear system, the portions of the system that are not linear vanish with decreasing distance from the critical point of interest faster than the linear portion vanishes. The linear portion (which can be expressed using a matrix equation with constant coefficients) dominates near the critical point. The cribsheet of linear stability analysis can be used to classify a critical point of an almost linear system with two modifications. If application of linear stability analysis suggests a star or a degenerate node, the shapes of the trajectories should be checked by carefully graphing by hand. If application of linear stability analysis suggests a center, actual trajectories will circulate, but they need to be carefully graphed by hand to determine whether they sink inward, expand outward, or are closed. 

Lecture 65  07:46  
In this and the following four videos, we present some concepts that can be used to design and recognize mathematical models that support oscillatory behavior. In this first video, we show that oscillations can be viewed as cyclic loops in a 2dimensional plane. One way to arrange for a pair of variables R and J to perform oscillations is to let the timederivative of each variable be proportional to the value of the other variable, with a negative sign in the coefficient of one of these differential equations. 

Lecture 66  03:42  
The angles at which nullclines pass through the phase plane (e.g. steep vs. shallow) determine the relative arrangement of regions in which quivers point in the topleft, bottomleft, bottomright, and topright directions. By modifying the slopes of nullclines, and thus the relatively positions of these regions, the qualitative dynamics of a dynamical system might be modified to support a stable star, a stable spiral, a closed loop, or even an unstable spiral. One way to understand how parameters affect trajectories is to understand how parameters affect the slopes that nullclines make when drawn in the phase plane. 

Lecture 67  02:31  
A spiral sink can be modified to support a closedloop trajectory if the system is modified so as to perform motion in the present that would, in the original dynamical system, have, instead, been performed at a previous time. 

Lecture 68  03:48  
A deterministc spiral sink that is highly skewed can support repeated oscillations when stochastic fluctuations kick the system out of the sink and onto a nearby region of rapid flow. 

Section 7: Physical oncology  
Lecture 69  01:54  
Interfaces between the physical sciences and oncology have become especially active in recent years owing, in part, to the Physical SciencesOncology Centers (PSOC) Network funded by the U.S. National Cancer Institute. While physical and mathematical scientists have historically contributed to instrumentation and technology development in the medical sciences, the PSOC network also promotes the application of physical sciences ways of thinking to understanding basic cancer biology and cancer therapy. 

Lecture 70  06:23  
The abstract organized into this and the following two videos highlights two recent papers from authors at the University of California, San Francisco working within the Princeton Physical Sciences Oncology Center. In this video, we review examples of ways that the timings of biochemical reactions can appear to be random.
† The authors dedicate this paper to Dr Barton Kamen who inspired its initiation and enthusiastically supported its pursuit. The research described in these articles was supported by award U54CA143803 from the US National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the US National Cancer Institute or the US National Institutes of Health. (C) 20122013 David Liao (lookatphysics.com) CCBYSA (license updated 2013 March 27). When distributing this set of three videos under the Creative Commons license, please cite the full journal references above (including authors and dois) as well as the citation information for this video collection: Title: Dynamic heterogeneity for the physical oncologist 

Lecture 71  04:54  
Stochastic fluctuations in the levels of intracellular molecules can lead to transitions between phenotypic states in individual cells. 

Lecture 72  08:10  
In the previous video, we asked whether phenotypic interconversion was a source of therapeutic failure or a therapeutic opportunity. In this video, we develop a graphical device, called a metronomogram, to understand that the dynamics of a phenotypically interconverting population (eventual reduction, expansion, or maintenance of population size) can depend on whether therapy is administered with sufficient time frequency. 

Section 8: Spatiallyresolved systems  
Lecture 73  05:46  
We use a simple lattice model of synchronous reproduction of annual plants to give an example of a kind of spatiallyresolved modeling that is easy to program into personal computers for routine study. This example happens to use a "winner takes all" replacement rule. See Nowak and May, Nature (1992) for an article describing spatial patterns that can arise when using a "winner takes all" model. In this video, we see that heterogeneous coexistence (as distinguished from homogeneous dominance by a single subpopulation) can sometimes be promoted by spatial localization. 

Section 9: Statistical physics  
Lecture 74  06:47  
Systems have states and energy levels Energy can be exchanged between parts of a world If the Hamiltonian of the world is timeindependent, the overall energy of the world is conserved Fundamental postulate of statistical mechanics: In an isolated system, all accessible microstates are accessed equally 

Lecture 75  02:42  
Notating the configurations of a world consisting of multiple parts Cartesian product 

Lecture 76  12:05  
Bath: many parts Number of ways to find the bath configured exponentially decays with increasing system energy Boltzmann factor 

Lecture 77  05:15  
The system energy most typically observed is the one that corresponds to the greatest number, W, of configurations of the world Ways (W), entropy (sigma), free energy (F), probability (P), partition function (Z), taking derivative of Z Maximizing ways of the world Maximizing entropy of the world Minimizing free energy of the system 

Lecture 78  04:09  
Introduction to ideal chain, exploring world configurations Purpose: Calculate average elongation of chain 

Lecture 79  08:46  
Writing the Hamiltonian for a series of independent links  
Lecture 80  17:00  
The average of the elongation of the chain (averaged by exploring states of the chain while the world explores accessible states equally) saturates for large weights  
Lecture 81  10:17  
In this unit, we provide intuitional background for studying Jeremy England's recent paper, "Statistical physics of selfreplication," at a level mostly appropriate for algebrabased high school physics courses. To understand the irreversibility of a macroscopic state change, it is important to compare the volumes of the portions of phase space corresponding to two macrostates within the volume of phase space that is kinetically accessible. In this video, we provide probabilistic language for describing the dynamic exploration of microstates of a universe. 

Lecture 82  05:06  
In the models we will consider, the conditional probability of a transition from a microstate of the universe, i, to a microstate universe, j, is equal to the conditional probability of a transition from microstate j to microstate i. We refer to this assumption as an assumption of microscopic reversibility. 

Lecture 83  11:08  
Transitions from a cluster of microstates of the universe associated with one microstate of the system to another cluster of microstates of the universe associated with another microstate of the system can be probabilistically favored to proceed in the forward direction. This occurs when the number of microstates in the final cluster is greater than the number of microstates in the initial cluster. Irreversibility equals the ratio of the number of microstates of the universe in the final cluster to the number of microstates of the universe in the initial cluster. Irreversibility increases with increasing heat exhausted to the reservoir when paths are taken in the forward direction. 

Lecture 84  11:44  
The irreversibility of a transition from a macroscopic state to another macroscopic state depends on the numbers of microstates of the universe in the two macrostates. The irreversibility of such a transition effectively equals the ratio of the number of kinetically accessible microstates of the universe belonging to the second macrostate of interest to the number of kinetically accessible microstates of the universe belonging to the first macrostate of interest. 

Section 10: Appendix: Algebra  
Lecture 85  04:07  
Caution: The videos in this appendix are more like "dinner talks" at research symposia than lectures from a crash course in algebra. There are not enough exercises, and some of the fancier ideas are not used elsewhere in the course. The purpose of decorating familiar ideas with slightly unfamiliar language is to inspire interest in finding out "what other ideas might I associate with mathematics that I initially thought was familiar?" In this and the following three videos, we will review the concept of quantity, which is represented by numbers. In this video, we review two ways in which we learned to think about numbers in elementary school. We used numbers to refer to the idea of having distinct manipulatives, and we used numbers to refer to the idea of labeling geographic locations with addresses. 

Lecture 86  04:40  
The analysis of a system of particles display BoseEinstein statistics is an example of a situation in which it is important to be aware whether we are thinking of numbers in terms of distinct manipulatives or in terms of addresses on a street. Incorrectly assuming that atomic and subatomic particles are just as distinct as the plastic counting manipulatives from kindergarten leads to overestimating the number of ways that particles can be excited out of the lowest energy state. In some situations, a system of particles that tends to occupy the lowest energy state in a way that is quantitatively consistent with thinking of numbers in terms of addresses (rather than thinking of particles as distinct manipulatives) is sometimes referred to as a BoseEinstein condensate. 

Lecture 87  02:27  
Numbers can be represented using a number line, a wedge, and placevalue representation. The application of memorized rules for performing arithmetic on numerals formatted in placevalue representation is called algorism. 

Lecture 88  01:42  
Infinity is not a number. There is no tick mark on the number line labeled "infinity." 

Lecture 89  05:42  
This slide deck presents aspects of quantitative "vocabulary" (variables) and quantitative "grammar" (functions and function composition) that will allow us to express quantitative reasoning in future slide decks. In this first of five videos, we note that it is cumbersome to describe quantitative relationships purely through the enumeration of repetitive examples involving concrete numbers. This difficult can be addressed with the assistance of abstract "placeholder," "standin" symbols. A variable is a symbol that stands in for a number at once arbitrary, yet specific and particular. Using variables, we can communicate quantitative relationships concisely. 

Lecture 90  04:45  
Functions are basic buildingblock sentences of mathematical reasoning. A function relates input values in a domain to output values in a codomain, and these associations can be depicted using plots. While different disciplines use slightly different definitions of a function, an essential stipulation familiar to scientists and mathematicians from a variety of fields is that a function associates each input value with precisely one output value. 
David's illustrations have been published in Science, Physical Review Letters, Molecular Pharmaceutics, Biosensors and Bioelectronics, and the Proceedings of the National Academy of Sciences.
University of California, San Francisco
Associate Professional Researcher 2015Current
Analyst, 20122014
Postdoc, 20102012 Tlsty Lab
Princeton University (PhD, Physics, 2010 MA, Physics, 2007)Advisor: Robert H. Austin
20062009 National Defense Science and Engineering Graduate Research Fellowship
20092010 National Science Foundation Graduate Research Fellowship
Harvey Mudd College BS, Physics, 2005
Advisor: Robert J. Cave