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A309181 Least number with exactly n nonunitary divisors. 4
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified September 27 13:48 EDT 2021. Contains 347688 sequences. (Running on oeis4.)