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A115907 Numbers k such that sigma(k) plus the k-th prime is a triangular number. 3
1, 2, 9, 13, 21, 80, 112, 129, 147, 225, 308, 349, 1063, 1282, 1300, 1635, 1880, 2686, 2956, 3517, 3759, 3907, 4058, 5025, 5196, 6742, 7606, 7708, 9244, 10154, 10596, 11259, 11719, 12687, 13167, 13599, 14081, 14624, 15937, 16437, 16766, 18289 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

David A. Corneth, Table of n, a(n) for n = 1..11241 (first 1000 from Robert Israel)

EXAMPLE

sigma(1880) + prime(1880) = 20503 = T(202).

MAPLE

filter:= n -> issqr(1+8*(numtheory:-sigma(n)+ithprime(n))):

select(filter, [$1..10^5]); # Robert Israel, Nov 07 2018

MATHEMATICA

Select[Range[25000], IntegerQ[(Sqrt[8(DivisorSigma[1, #]+Prime[#])+ 1]-1)/2]&] (* Harvey P. Dale, Jun 07 2011 *)

PROG

(PARI) upto(n) = my(t = 0, res = List(), u = prime(n)); forprime(p = 2, u, t++; c = p + sigma(t); if(ispolygonal(c, 3), listput(res, t))); res \\ David A. Corneth, Nov 08 2018

CROSSREFS

Cf. A000040, A000203, A000217, A115882, A115908.

Sequence in context: A234902 A138946 A037385 * A063634 A178688 A163735

Adjacent sequences:  A115904 A115905 A115906 * A115908 A115909 A115910

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved

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Last modified February 27 04:35 EST 2020. Contains 332299 sequences. (Running on oeis4.)