A189974 - OEIS

A189974

Numbers n such that d(n-1) = d(n+1) = 4, where d(k) is the number of divisors of k (A000005).

7, 9, 34, 56, 86, 92, 94, 124, 142, 144, 160, 184, 186, 202, 204, 214, 216, 218, 220, 236, 248, 266, 300, 302, 304, 320, 322, 328, 340, 342, 392, 394, 412, 414, 416, 446, 452, 470, 472, 516, 518, 534, 536, 544, 552, 580, 582, 590, 634, 668, 670, 680, 686

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OFFSET

1,1

COMMENTS

Numbers n such that n-1 and n+1 are both multiplicatively perfect numbers A007422.

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

MAPLE

with(numtheory): A189974 := proc(n) option remember: local k: if(n=1)then return 7:else k:=procname(n-1)+1: do if(tau(k-1)=4 and tau(k+1)=4)then return k: fi: k:=k+1: od: fi: end: seq(A189974(n), n=1..60); # Nathaniel Johnston, May 04 2011

MATHEMATICA

Select[Range[2, 754], DivisorSigma[0, # - 1] == DivisorSigma[0, # + 1] == 4 &]

CROSSREFS

Cf. A000005, A007422.

Sequence in context: A272431 A261961 A177030 * A316184 A359367 A321760

Adjacent sequences: A189971 A189972 A189973 * A189975 A189976 A189977

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, May 03 2011

STATUS

approved