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Re: Interesting failure of Collect

• To: mathgroup at smc.vnet.net
• Subject: [mg61343] Re: [mg61307] Interesting failure of Collect
• From: "David Park" <djmp at earthlink.net>
• Date: Sun, 16 Oct 2005 00:17:58 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

Jerry,

It looks like a bug but it is probably an unintuitive feature from trying to
extend the Collect patterns to include numbers. Pi seems to be permitted but
the 1/4 causes the problem. You may be able to overcome the problem by
omitting numbers and including FactorTerms as the last argument in Collect.

Collect[expr, Sin[p]/Pi, FactorTerms]

((1/a + 1/b)*Sin[p])/(4*Pi)

Collect[expr2, Sin[p]/Pi, FactorTerms]

((1/a - 1/b)*Sin[p])/(4*Pi)

The last argument to Collect is easy to overlook and is often quite useful.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/




From: Blimbaum, Jerry CIV NSWC PC [mailto:jerry.blimbaum at navy.mil]
To: mathgroup at smc.vnet.net


Given the expr =  Sin[p]/(4 Pi a) + Sin[p]/(4 Pi b)

and then applying the command


Collect[expr, Sin[p]/(4 Pi)] works as it should, however,

expr2 = Sin[p]/(4 Pi a) - Sin[p]/(4 Pi b)

leaves the expression untouched.   (I realize I could use Simplify but
this problem occurred on a much longer expression and this is intended
just to convey the basic idea)....for longer expressions the Collect
process works for all quantities with a + sign but the one with a minus
sign will not be collected.....strikes me as a "bug".....


thanks....jerry blimbaum