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A038456 List of pairs of consecutive numbers each with 4 divisors (duplicates removed). 4
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 (list; graph; refs; listen; history; text; internal format)
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 * A075658 A047821 A195238

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

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Last modified September 18 18:44 EDT 2019. Contains 327180 sequences. (Running on oeis4.)