Re: DSolve Question

*To*: mathgroup at smc.vnet.net*Subject*: [mg50602] Re: [mg50589] DSolve Question*From*: Selwyn Hollis <sh2.7183 at misspelled.erthlink.net>*Date*: Sun, 12 Sep 2004 04:42:13 -0400 (EDT)*References*: <200409111044.GAA19893@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

On Sep 11, 2004, at 6:44 AM, John Reed wrote: > I'm relatively new to Mathematica, and I'm having trouble with a > simple DE. > This is from Zimmerman & Olness, "Mathematica for Physics". Here's the > equation: > > eq1 = y''[t] + w^2 y[t] == 0 > > initial = {y[0] == y0, y'[0] == v0} > > eq3 = DSolve[{eq1, initial}, y[t], t] > > When this executes, I get the error message bvnul: For some branches > of the > general solution, the given boundary conditions lead to an empty > solution. > > Now, if I change v0 to a number, everything works fine. I've tried > making > v0 real and positive, but with no luck. > > Any help is appreciated. > > John Reed Hi John, You've rediscovered a "bug" in DSolve that's new in 5.0. Here's one workaround: eq1 = y''[t] + w^2 y[t] == 0 initial = {y[0] == y0, y'[0] == v0} soln[t_] = y[t] /. First@DSolve[eq1, y[t], t] soln[t] /. First@Solve[initial /. y -> soln, {C[1], C[2]}] ----- Selwyn Hollis http://www.appliedsymbols.com (edit reply-to to reply)

**References**:**DSolve Question***From:*"John Reed" <nospamjreed@alum.mit.edu>