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Re: Surprising DSolve problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg123986] Re: Surprising DSolve problem
  • From: "Scot T. Martin" <smartin at seas.harvard.edu>
  • Date: Wed, 4 Jan 2012 05:01:25 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201201020742.CAA09063@smc.vnet.net>

I don't promise that I am right on this but I take a hunch based on similar experiences with Integrate[...]. The hunch is that at times Mathematica is "trying too hard". That means that it allows for a general case that the coefficient 'a' can be an imaginary number, negative, and so forth. In this case, set up your expression using Assume[...] and put in there Element[a,Reals] or "a > 0" as needed. In any case, I did not test this on your example, so it is somewhat of a shot in the dark. 


________________________________________
From: Sam Takoy [sam.takoy at yahoo.com]
Sent: Monday, January 02, 2012 02:42
To: mathgroup at smc.vnet.net
Subject: [mg123986] Surprising DSolve problem

Hi,

I find that my Mathematica (8.0.1)

can do

DSolve[ y''[x] + (2 y[x])/(Cosh[x]^2) == 0, y, x]

but can't do

DSolve[ y''[x] + (2 y[x])/(a^2 Cosh[x/a]^2) == 0, y, x] (*Additional
constant "a")

If I am not mistaken, the equations are equivalent by a trivial change
of variables.

What's going on here?

Thanks,

Sam




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