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%I #19 Sep 08 2022 08:46:09
%S 2,3,5,11,23,29,41,53,83,89,113,131,173,179,191,233,239,251,281,293,
%T 329,359,413,419,431,443,491,509,593,623,641,653,659,683,719,743,761,
%U 809,869,911,953,979,1013,1019,1031,1049,1103,1223,1229,1289,1409,1439,1451
%N Numbers k such that sigma(k + sigma(k)) = 2*sigma(k).
%C Union of A005384 (Sophie Germain primes) and A246858.
%C First composite number in sequence is 329 (see A246858).
%e Composite number 329 (with sigma(329) = 384) is in sequence because sigma(329+sigma(329)) = sigma(713) = 768 = 2*384.
%e Prime 359 (with sigma(359) = 360) is in sequence because sigma(359+sigma(359)) = sigma(719) = 720 = 2*360.
%t Select[Range[1500], DivisorSigma[1, # + DivisorSigma[1, #]] == 2 DivisorSigma[1, #] &] (* _Michael De Vlieger_, Aug 05 2021 *)
%o (Magma) [n:n in[1..10000] | SumOfDivisors(n+SumOfDivisors(n)) eq 2*SumOfDivisors(n)]
%o (PARI) select(n -> sigma(n+sigma(n))==2*sigma(n),[1..1000]) \\ _Edward Jiang_, Sep 05 2014
%Y Cf. A074400, A246456.
%Y Cf. A005384, A246858.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Sep 05 2014
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