A309181 Least number with exactly n nonunitary divisors.

1, 4, 8, 16, 24, 36, 48, 256, 72, 1024, 180, 144, 216, 16384, 288, 65536, 360, 576, 3072, 900, 864, 1296, 720, 2304, 1080, 67108864, 2592, 268435456, 1440, 9216, 196608, 5184, 2160, 17179869184, 2880, 36864, 10368, 3600, 6300, 1099511627776, 4320, 20736, 6480

Offset

0,2

Links

Amiram Eldar, Table of n, a(n) for n = 0..100

Formula

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

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

Mathematica

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

Prog

(PARI) nnud(n) = numdiv(n) - 2^omega(n); \\ A048105
a(n) = my(k=1); while (nnud(k) != n, k++); k; \\ Michel Marcus, Jul 17 2019

Crossrefs

Cf. A005179, A048105, A309141.

Sequence in context: A137932 A309141 A329882 * A140466 A343421 A337353

Adjacent sequences: A309178 A309179 A309180 * A309182 A309183 A309184

Keyword

nonn

Author

Amiram Eldar, Jul 15 2019

Status

approved