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MathGroup Archive 2002

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Re: The prime factors of n.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32700] Re: [mg32673] The prime factors of n.
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Thu, 7 Feb 2002 05:09:24 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Certainly not. For example

f1[n_Integer] := Flatten[Table[#1, {#2}] & @@@ FactorInteger[n]]

In[2]:=
f1[72]

Out[2]=
{2,2,2,3,3}

is both more elegant and faster.

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Wednesday, February 6, 2002, at 05:41  PM, Robert G. Wilson v wrote:

> Hello all,
>
>         I wish to receive a list of prime factors of n not in the form
> returned by FactorInteger. Instead I want only the primes the number of
> times they appear. As an example I will use 72. FactorInteger[72] gives
> { {2,3}, {3,2} }. I wish the list would read { 2, 2, 2, 3, 3 }. Is the
> following the best that I can do? f[n_Integer] := Module[{a =
> FactorInteger[n], b = {}}, While[Length[a] > 0, Do[b = Append[b, a[[1,
> 1]]], {a[[1, 2]]}]; a = Drop[a, 1]]; b] .
>
> See
> http://www.research.att.com/cgi-
> bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=037276
>
> Sincerely yours,
>
> Robert G. "Bob" Wilson, V
>
>
>
>



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