A309181 - OEIS

%I #17 Jul 18 2019 07:44:48

%S 1,4,8,16,24,36,48,256,72,1024,180,144,216,16384,288,65536,360,576,

%T 3072,900,864,1296,720,2304,1080,67108864,2592,268435456,1440,9216,

%U 196608,5184,2160,17179869184,2880,36864,10368,3600,6300,1099511627776,4320,20736,6480

%N Least number with exactly n nonunitary divisors.

%H Amiram Eldar, <a href="/A309181/b309181.txt">Table of n, a(n) for n = 0..100</a>

%F A048105(a(n)) = n and A048105(k) != n for all k < a(n).

%F a(n) <= 2^(n+1).

%t nud[n_] := DivisorSigma[0, n] - 2^PrimeNu[n]; m = 24; c = 0; s = Table[0, {m + 1}]; k = 1; While[c < m + 1, n = nud[k]; If[n <= m && s[[n + 1]] == 0, c++; s[[n + 1]] = k]; k++]; s

%o (PARI) nnud(n) = numdiv(n) - 2^omega(n); \\ A048105

%o a(n) = my(k=1); while (nnud(k) != n, k++); k; \\ _Michel Marcus_, Jul 17 2019

%Y Cf. A005179, A048105, A309141.

%K nonn

%O 0,2

%A _Amiram Eldar_, Jul 15 2019