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

**References**:**Surprising DSolve problem***From:*Sam Takoy <sam.takoy@yahoo.com>