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A038456
List of pairs of consecutive numbers each with 4 divisors (duplicates removed).
5
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
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
Sequence in context: A278979 A220671 A136012 * A346549 A075658 A047821
KEYWORD
nonn
AUTHOR
EXTENSIONS
Corrected and extended by Olivier Gérard
Corrected by Rick L. Shepherd, Jun 07 2002
STATUS
approved