Second order, nonlinear differential equation help

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The discussion centers on solving the second-order nonlinear differential equation y'' + t*y + y*y' = sin(t). A participant suggests using a power series solution to tackle the problem, while another provides Mathematica code that outlines the approach. The code includes calculations for both the left and right sides of the equation, leading to a numerical solution using NDSolve. The resulting plots illustrate the behavior of the solution over a specified range. Overall, the conversation highlights the complexity of the equation and offers a computational method for finding a solution.
cpyles1
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I've been trying to solve this one for a while. My professor wasn't even sure how to do it. Any suggestions?
y''+t*y+y*y'=sin(t)
 
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cpyles1 said:
I've been trying to solve this one for a while. My professor wasn't even sure how to do it. Any suggestions?
y''+t*y+y*y'=sin(t)

Yeah. How about a power series solution? Huh? What do you mean that yy' thing? No? How about the Mathematica code below then. Looks pretty close to me.

Code:
Remove[a];

nmax = 25; 

myleftside = Sum[n*(n - 1)*Subscript[a, n]*t^(n - 2), {n, 0, nmax}] + 
    Sum[Subscript[a, n - 3]*t^(n - 2), {n, 3, nmax + 2}] + 
    Sum[Subscript[a, k]*Subscript[a, n - k]*(n - k)*t^(n - 1), {n, 0, nmax + 1}, {k, 0, n}]; 

myrightside = Sum[((-1)^n*t^(2*n + 1))/(2*n + 1)!, {n, 0, nmax}]; 

myclist = Flatten[Table[Solve[Coefficient[myleftside, t, n] == Coefficient[myrightside, t, n], 
      Subscript[a, n + 2]], {n, 0, nmax}]]; 

Subscript[a, 0] = 0; 
Subscript[a, 1] = 1; 

mysec = Table[Subscript[a, n] = Subscript[a, n] /. myclist, {n, 2, nmax}]; 

thef[t_] := Sum[Subscript[a, n]*t^n, {n, 0, nmax}];

p1 = Plot[thef[t], {t, 0, 2}, PlotStyle -> Red];

mysol = NDSolve[{Derivative[2][y][t] + t*y[t] + y[t]*Derivative[1][y][t] == Sin[t], y[0] == 0, 
    Derivative[1][y][0] == 1}, y, {t, 0, 2}];

p2 = Plot[y[t] /. mysol, {t, 0, 2}, PlotStyle -> Blue];

Show[{p1, p2}]
 
Last edited:

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