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Number of permutations of the multiset of prime factors of n whose first part is not 2.
3

%I #10 Dec 07 2021 11:08:26

%S 1,0,1,0,1,1,1,0,1,1,1,1,1,1,2,0,1,2,1,1,2,1,1,1,1,1,1,1,1,4,1,0,2,1,

%T 2,3,1,1,2,1,1,4,1,1,3,1,1,1,1,2,2,1,1,3,2,1,2,1,1,6,1,1,3,0,2,4,1,1,

%U 2,4,1,4,1,1,3,1,2,4,1,1,1,1,1,6,2,1,2,1,1,9,2,1,2,1,2,1,1,2,3,3,1,4,1,1,6

%N Number of permutations of the multiset of prime factors of n whose first part is not 2.

%H Antti Karttunen, <a href="/A325392/b325392.txt">Table of n, a(n) for n = 1..20000</a>

%F If n is odd, a(n) = A008480(n). If n is even, a(n) = A008480(n) - A008480(n/2).

%e The a(90) = 9 permutations of {2,3,3,5} not starting with 2:

%e 3 2 3 5

%e 3 2 5 3

%e 3 3 2 5

%e 3 3 5 2

%e 3 5 2 3

%e 3 5 3 2

%e 5 2 3 3

%e 5 3 2 3

%e 5 3 3 2

%t primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];

%t Table[Length[Select[Permutations[primeMS[n]],#=={}||First[#]>1&]],{n,100}]

%o (PARI)

%o A008480(n) = {my(sig=factor(n)[, 2]); vecsum(sig)!/factorback(apply(k->k!, sig))}; \\ From code in A008480

%o A325392(n) = if(n%2, A008480(n), A008480(n)-A008480(n/2)); \\ _Antti Karttunen_, Dec 06 2021

%Y Number of times n appears in A325390.

%Y Cf. A008480, A056239, A112798, A325327, A325362, A325364, A325367, A325403 (even bisection), A325407, A325460, A325461.

%K nonn

%O 1,15

%A _Gus Wiseman_, May 02 2019

%E Data section extended up to 105 terms by _Antti Karttunen_, Dec 06 2021

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Last modified September 23 07:29 EDT 2024. Contains 376143 sequences. (Running on oeis4.)