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A115909 Numbers n such that sigma(n) times n is a triangular number. 2

%I

%S 1,2,4,8,16,32,57,64,65,85,87,128,256,512,1024,1245,1421,2048,3146,

%T 3330,3790,4096,6695,7257,8192,10137,16384,25884,32768,34420,34551,

%U 34947,65536,131072,208495,262144,348161,440495,524288,530270,534430

%N Numbers n such that sigma(n) times n is a triangular number.

%C Infinite, since all the powers of 2 belong to the sequence.

%e sigma(1421)*1421=2429910=T(2204).

%t Select[Range[540000],IntegerQ[(Sqrt[1+8(#*DivisorSigma[1,#])]-1)/2]&] (* _Harvey P. Dale_, Dec 16 2011 *)

%Y Cf. A115904, A115910.

%K nonn

%O 1,2

%A _Giovanni Resta_, Feb 06 2006

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Last modified May 31 13:01 EDT 2020. Contains 334748 sequences. (Running on oeis4.)