A038456 List of pairs of consecutive numbers each with 4 divisors (duplicates removed).

14, 15, 21, 22, 26, 27, 33, 34, 35, 38, 39, 57, 58, 85, 86, 87, 93, 94, 95, 118, 119, 122, 123, 133, 134, 141, 142, 143, 145, 146, 158, 159, 177, 178, 201, 202, 203, 205, 206, 213, 214, 215, 217, 218, 219, 253, 254, 298, 299, 301, 302, 303, 326, 327, 334, 335, 381, 382, 393, 394, 395, 445, 446, 447, 453, 454

Offset

1,1

References

D. Wells, Curious and interesting numbers, Penguin Books.

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

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

Mathematica

Union[Flatten[Select[Partition[Range[500], 2, 1], DivisorSigma[0, First[#]] == DivisorSigma[0, Last[#]]==4&]]] (* Harvey P. Dale, Jul 22 2012 *)
SequencePosition[DivisorSigma[0, Range[500]], {4, 4}]//Flatten//Union (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 15 2016 *)

Prog

(PARI) isA038456(n) = (numdiv(n)==4) && ((numdiv(n+1)==4) || (numdiv(n-1)==4)) \\ Michael B. Porter, Feb 03 2010

Crossrefs

Cf. A039832, A039833.

Sequence in context: A278979 A220671 A136012 * A346549 A075658 A047821

Adjacent sequences: A038453 A038454 A038455 * A038457 A038458 A038459

Keyword

nonn

Author

Felice Russo

Extensions

Corrected and extended by Olivier Gérard

Corrected by Rick L. Shepherd, Jun 07 2002

Status

approved