       Basic questions on _Plus and recursion

• To: mathgroup at smc.vnet.net
• Subject: [mg120861] Basic questions on _Plus and recursion
• From: Ethan Dyer <ethansdyer at gmail.com>
• Date: Sat, 13 Aug 2011 06:49:04 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

```Hi. I'm sure these are common issues, but I haven't found them. Here are the two, unrelated questions. Thank you for the help.

1) I want to use _Plus for a function with arbitrary arguments. ie I want: f[a+b]=f[a]+f[b], f[a+b,c+d] to go to f[a,c]+f[a,d]+f[b,c]+f[b,d], ... Is there a standard way to do this?

2)Here I want to pull out scalars from a function. Something like this:

f[x__] :=(Times@@Select[{x},Scalar[#]==1&]) (f@@Select[{x},Scalar[#]!=1&])

Where i have defined a function Scalar to =1 for some symbols.

for instance say I had defined Scalar[a]=1 then f[a,b,c]= a f[b,c].

The above does not work, as it gives infinite recursion. Is there a way to do this properly?

Thank you again. Sorry if these are too common.

Ethan

```

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