A079432 -id:A079432 - cp's OEIS frontend

A079431 Least m>n with same smallest odd prime factor as n.

2, 4, 6, 8, 10, 9, 14, 16, 12, 20, 22, 15, 26, 28, 18, 32, 34, 21, 38, 25, 24, 44, 46, 27, 35, 52, 30, 49, 58, 33, 62, 64, 36, 68, 40, 39, 74, 76, 42, 50, 82, 45, 86, 88, 48, 92, 94, 51, 56, 55, 54, 104, 106, 57, 65, 77, 60, 116, 118, 63, 122, 124, 66, 128, 70, 69, 134, 136

Offset: 1

Reinhard Zumkeller, Jan 09 2003

A078701(a(n))=A078701(n), A078701(i)<>A078701(n), n

n + A078701(n) <= a(n) <= 2*n;

a(2^k) = 2^(k+1), as A078701(2^k) = 1.

Robert Israel, Table of n, a(n) for n = 1..10000

Cf. A079432(n) = a(a(n)).

Cf. A078701.

N:= 100:
A:= Vector(N):
p:= 2:
do
  p:= nextprime(p);
  if p > N then break fi;
  R:= select(k -> A[k] = 0, [$1..N]);
  S:= select(`<=`,R,N/p);
  A[p*S]:= Vector(p*R[2..1+nops(S)]);
od:
for k from 0 to ilog2(N) do A[2^k]:= 2^(k+1) od:
convert(A,list); # Robert Israel, Oct 25 2017

sopf[n_] := SelectFirst[FactorInteger[n][[All, 1]], # > 2&];
a[n_] := For[p = sopf[n]; m = n+1, True, m++, If[p == sopf[m], Return[m]]];
Array[a, 100] (* Jean-François Alcover, Mar 09 2019 *)

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A079431 Least m>n with same smallest odd prime factor as n.

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