### Class diary and homework assignments: M231 MWF 9:05-9:55

W Aug 24: Intro. What *is* an ODE (a PDE)? order; linear/nonlinear Hwk 1.1: 1-12 due Fri
F Aug 26: Solution concept; IVP; radioactive decay; Picard-Lindelöf Hwk 1.1: 13,14,15; 1.2: 1,3,5, due Mon
M Aug 29: Example Picard-Lindelöf & Peano; implicit solution; direction fields. Hwk 1.2: 9,10,15,19,20a,21a,22a,23,25,26,29 and 1.3: 1,3,5,7 due Fri if possible
W Aug 31: Euler's approximation method; separable ODEs. Hwk: from notes pg3 fill in table. Due Fri. Hwk from Mon until 20a is definitely due Fri; the rest from Monday may be turned in Wed, if you prefer. But on Fri, there will come other hwk due Wed.
F Sep 02: The example y'=1+y2: finite time blow-up; Sol's to IVP's are defined on *intervals* only. --- First outlook on linear 1st order ODEs. Hwk: 2.2: 1-5,7,12,17-19,21; 27b,29,31a-c due Wed. In 17-19,21, make sure IN ADDITION to what the book asks, to give the *interval* on which sol's are defined.
M Sep 05: LABOR DAY
W Sep 07: Linear 1st order ODEs: finding and using an integrating factor. New Hwk: 2.3: 1-6, 8,11 due Fri. 16-18, 20, 25a, 28 due Mon --- Deadline for old hwk 2.2: 21, 27b,29,31a-c extended till Friday
F Sep 09: Velocity as an integrating factor in Newtonian mechanics; homogeneous ODEs and the v=y/x substitution, and one example.
M Sep 12: More substitutions (in particular Bernoulli). Another example like 2.2,27: How to use *definite* integrals to get the right constant for IVP's when the integral cannot be evaluated practically. Hwk: 2.6: 10,13,18,21,22 due Wed -- Review pblms pg81: 1,2,4,8,9,15,17,32. Give interval of existence with 32; due Fri -- also scan all the review pblms to see in which of the studied categories (if any) they fit. This latter part is not for turn in
W Sep 14: Modelling: mixing problems: a first view of the superposition principle. Hwk: 3.2: 1,3,4,5,8 due Mon. --- Extension given for 2.6 hwk till Fri
F Sep 16: Example for superposition principle in the solution of a mixing problem's ODE. -- Population models; heating/cooling --- Hwk: 3.2: 9,14,19,27 due Mon
M Sep 19: Q&A; Modelling with Newton's equations if forces depend on velocity only. Hwk: Sec 3.3: 1,3,5,7,12,15. Sec 3.4: 3 --- sol's will be posted but hwk will not be turned in for grading, so you have the feedback before the exam
W Sep 21: Using the Newton method to determine hit-ground time, and when we get it cheaper. Dimensionless time variable. Quick overview for exam. The superposition principle for linear ODEs started.
F Sep 23: Outline Linear ODEs: splitting ``general solution of lin'inhom = general solution of lin'hom plus particular sol of lin'inhom''; one example to illustrate this. Hwk: You won't turn this in, just do it by next Fri: Evaluate the complex-number expression (2+i)/(3-4i) + (2-i)/(3+4i). Also evaluate (cos a + i sin a)(cos b + i sin b), using trig identities to simplify the result.
M Sep 26: ****** EXAM 1 ******
W Sep 28: Exam back, and discussion. Exponential ansatz for lin'hom' ODE with const coeffs. Why we get *all* solutions from linear combinations of this ansatz. Hwk: Ch4.2: 1-4,7,8,13,14, 41,42,48 due Mon. Postpone 3,4 until after Friday's class, all the rest you can do now. Also remember last Friday's hwk, and do it by this Friday.
F Sep 30: Linear independence, and the Wronskian. Double / multiple roots of the characteristic equation and how the solution arises in the limit of coalescing single roots.
M Oct 03: Complex roots of the characteristic equation, and Euler's formula: Hwk Ch4.3: 10, 20, 22, 32, 33, 35, 37b. Due Fri (Try some more of the smaller numbers 1-20 without turn-in, if you need more practice.)
W Oct 05: Examples on complex calculations, including (for later use) integral of exponential*(sin or cos). Sneak preview of method of undetermined coefficients
F Oct 07: Method of undetermined coefficients. Hwk: Ch 4.4: 1-8, 10,11,13,16,17, 29-31,34. Ch 4.5: 25,28,30 due Wed
M Oct 10: Method of undetermined coefficients; ramifications. An example on piecewise defined rhs introduced. Hwk: Ch.4.5: 32-37, 39, 45 due Wed or Mon
Link to extra credit problems
W Oct 12: Introduction to Variation of Parameters: comparison with 1st order, and one full-size example. Hwk: Ch4.6, 1,6,10,22,24 due Fri next week
F Oct 14: FALL BREAK
M Oct 17: Comments on IVP for inhom. ODEs: determine C1, C2 by plugging initial values into solution of inhomogeneous ODE! --- Another example on varition of parameters (3rd order; from book p 340, but solving for v1, v2, v3 the pedestrian way. (remember) Hwk: Ch4.6, 1,6,10,22,24 due Fri
W Oct 19: Writing A cos x + B sin x as R sin(x+phi). The free oscillator: discussion, calculation, interpretation.
F Oct 21: Informal background info on periodic forcings, which can be written as superpositions of trigs. Forced oscillator: complex calculation
M Oct 24: Forced oscillator: Interpreatation and Resonance phenomena
W Oct 26: Phase shift and resonance; Review on variation of parameters
F Oct 28: brief introduction to Laplace transform methods; Calculating a LT from scratch.
M Oct 31: EXAM 2
W Nov 02: Exam back. Linearity and other simple properties of the LT. Hwk: Ch 7.2: 2,3,4,6,8,10,11,14,20,31 due Fri or Mon
F Nov 04: More properties of the LT. Solving a simple IVP via LT. Can two different fcts have the same LT? (in principle yes, but for practical purposes: no - not two different CONTINUOUS fcts!) Hwk Ch 7.3: 2,4,6,8,10,12,14,16,18; 31 due Mon or Wed
M Nov 07: Is every F(s) somebody's LT? (No way. However those F(s) we get from attempting to solve IVP's are.) Intro to Partial Fraction Decomposition.
W Nov 09: PFD: Cover-up method and its variants in the presence of multiple, or complex roots. Hwk 7.4: 2,4,6,8,10,15,16, 21,22,27,28, 33,36 due Fri or Mon
F Nov 11: Solving IVP's via LT: primarily const coeff's; plus an example where coeff's are no worse than at+b (leading to a 1st order ODE for Y(s)). Hwk 7.5: 3,4,7,8,23,28; 37,38 due Wed
M Nov 14: Q& A; another refinement on PFD (pg 6,7 item (ii) of PFD notes). The unit step function, and how to do LT's of piecewise defined functions. Hwk 7.6: 1-5,9,12,14,16,18,27,28. This hwk will not be graded, so that you don't need to relinquish your solutions during exam prep. But you should do it before the exam, and solutions will be posted for your comparison. 27 and 28 needs Wed's class yet
Nov 15: Last day to drop with WP/WF
W Nov 16: Solving an IVP with piecewise def'd rhs (7.6 pblm 39 carried through in all detail)
F Nov 18: Review (for future use) the geometric series. -- Q&A -- TCE
M Nov 21: EXAM 3
W Nov 23:
F Nov 25: THANKSGIVING
M Nov 28:
W Nov 30:
F Dec 02:
M Dec 05:
W Dec 07: STUDY PERIOD

M Dec 12: FINAL EXAM 10:15-12:15