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%I #7 Jul 13 2015 05:18:06
%S 2,38,66290,113978,9121109348
%N Numbers n such that sigma(n+2)-2sigma(n+1)+sigma(n) = n.
%C The equation here is the difference equation (applied to sigma) corresponding to the differential equation y" = x (Hooke's law with constant = -1).
%C a(6) > 10^13. - _Giovanni Resta_, Jul 13 2015
%e sigma(40)-2*sigma(39)+sigma(38) = 90 - 2*56 + 60 = 38, so 38 is a term of the sequence.
%t Select[Range[1, 10^6], DivisorSigma[1, # + 2] - 2*DivisorSigma[1, # + 1] + DivisorSigma[1, # ] == # &]
%K more,nonn
%O 1,1
%A _Joseph L. Pe_, Dec 19 2001
%E a(5) from _Donovan Johnson_, Feb 01 2009
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