A131904 - OEIS

%I #7 Apr 03 2015 03:47:39

%S 2,2,2,6,4,6,2,2,6,6,2,12,2,12,6,6,2,4,6,6,12,2,24,2,12,6,6,6,2,6,6,

%T 24,2,24,2,12,12,6,2,4,12,6,12,2,24,6,24,6,6,2,2,6,12,6,24,2,12,6,24,

%U 2,60,2,6,12,12,6,24,2,48,16,6,2,60,6,6,6,24,2

%N Smallest positive integer k with the same number of divisors as the n-th integer for which such a k exists.

%F a(n)=min(k>0, k has the same number of divisors as A131903(n))

%e a(4)=6 because A131903(4)=8, which has four divisors, and 6 is the least positive integer with four divisors

%t Clear[tmp]; Function[n, If[Head[ #1] === tmp, #1 = n; Unevaluated[Sequence[]], # ] & [tmp[DivisorSigma[0, n]]]] /@ Range[64]

%o (PARI) lista(nn) = {for (n=1, nn, my(nd = numdiv(n)); for (k=1, n-1, if (numdiv(k) == nd, print1(k, ", "); break);););} \\ _Michel Marcus_, Apr 03 2015

%Y Cf. A069822, A131902-A131908.

%K easy,nonn

%O 1,1

%A Peter Pein (petsie(AT)dordos.net), Jul 26 2007

%E More terms from _Michel Marcus_, Apr 03 2015