A039832 - OEIS

A039832

Numbers k such that k and k+1 both have 4 divisors.

14, 21, 26, 33, 34, 38, 57, 85, 86, 93, 94, 118, 122, 133, 141, 142, 145, 158, 177, 201, 202, 205, 213, 214, 217, 218, 253, 298, 301, 302, 326, 334, 381, 393, 394, 445, 446, 453, 481, 501, 514, 526, 537, 542, 553, 565, 622, 633, 634, 694, 697, 698, 706, 717, 745, 766, 778, 793, 802, 817

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OFFSET

1,1

REFERENCES

David Wells, Curious and interesting numbers, Penguin Books, 1986, p. 91.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Vincenzo Librandi)

EXAMPLE

14 and 15 both have 4 as number of divisors and are consecutive.

MATHEMATICA

Flatten[Position[Partition[Table[DivisorSigma[0, n], {n, 1000}], 2, 1], _?(#=={4, 4}&)]] (* Vincenzo Librandi, Oct 21 2012 *)

PROG