Re: wrong solution for double integral of piecewise function?

*To*: mathgroup at smc.vnet.net*Subject*: [mg96987] Re: wrong solution for double integral of piecewise function?*From*: Tom Roche <tlroche at gmail.com>*Date*: Sun, 1 Mar 2009 04:55:37 -0500 (EST)*References*: <gob80u$g9q$1@smc.vnet.net>

Tom Roche Feb 28, 6:43 am >> Solve[ >> First[ >> Integrate[ >> Integrate[ >> f[\[Chi], \[Psi]], {\[Psi], -\[Infinity], \[Infinity]} >> ], >> {\[Chi], -\[Infinity], \[Infinity]} >> ] >> ] == 1, k >> ] >> [gets] >> (2) {{k -> -(1/((a - b)^2 (-1 + UnitStep[a - b])))}} Bob Hanlon > When you integrated by hand you assumed that b > a, Mathematica > gives a more complicated result since it does not make that > assumption unless you tell it to do so. and he points out that the proper way to do this is by wrapping an Assuming around the procedure above: Assuming[{b > a}, Solve[ First[ Integrate[ Integrate[ f[\[Chi], \[Psi]], {\[Psi], -\[Infinity], \[Infinity]} ], {\[Chi], -\[Infinity], \[Infinity]} ] ] == 1, k ] ] produces {{k -> 1/(a - b)^2}} Thanks!