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 A236387 Numbers n such that sigma(n) is an oblong number. 2

%I

%S 5,6,11,19,20,26,28,29,30,39,40,41,46,51,55,58,71,86,89,99,104,109,

%T 114,116,117,125,131,135,158,177,181,201,202,203,209,216,226,236,239,

%U 245,271,278,306,336,340,352,377,379,398,410,411,419,428,442,447,461

%N Numbers n such that sigma(n) is an oblong number.

%C An oblong number (A002378) is of the form k(k+1) where k is a natural number.

%C The subsequence of prime terms is A002327 (primes of form n^2 - n - 1). - _Michel Marcus_, Jan 09 2015

%H Giovanni Resta, <a href="/A236387/b236387.txt">Table of n, a(n) for n = 1..10000</a>

%e sigma(40) = 90 = 9*10, an oblong number; so 40 is a term of the sequence.

%t Select[Range[500], IntegerQ@ Sqrt[1+4*DivisorSigma[1, #]] &] (* _Giovanni Resta_, Jan 24 2014 *)

%Y Cf. A000203, A002327, A002378.

%K nonn,easy

%O 1,1

%A _Joseph L. Pe_, Jan 24 2014

%E a(12)-a(56) from _Giovanni Resta_, Jan 24 2014

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Last modified December 9 17:18 EST 2019. Contains 329879 sequences. (Running on oeis4.)