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%I #13 Feb 09 2020 08:39:11
%S 56,120,144,160,176,186,204,208,216,220,246,260,288,300,304,320,324,
%T 340,342,364,392,414,416,426,474,476,496,516,528,532,534,544,550,552,
%U 560,580,582,624,636,650,666,680,696,704,714,736,748,780,784,792,800
%N Abundant or perfect numbers k such that neither k-1 nor k+1 is a prime.
%H Amiram Eldar, <a href="/A047779/b047779.txt">Table of n, a(n) for n = 1..10000</a>
%H <a href="https://web.archive.org/web/20011215180950/http://bruichladdich.dcs.st-and.ac.uk:80/maths/abundants.html">Haskell Program for Abundant and Perfect Numbers</a> [Cached copy at the Wayback Machine].
%e a(1) = 56 because 55 and 57 are composite and the sum of the divisors of 56 is 64 which is >= 56 and no integer < 56 has this property.
%t Select[Range[800], DivisorSigma[1, #] >= 2 # && And @@ CompositeQ[# + {-1, 1}] &] (* _Amiram Eldar_, Feb 09 2020 *)
%o (PARI) isok(n) = (sigma(n) >= 2*n) && ! isprime(n-1) && ! isprime(n+1) \\ _Michel Marcus_, Jun 12 2013
%Y A023196 is a superset.
%K easy,nonn
%O 1,1
%A Tony Davie (ad(AT)dcs.st-and.ac.uk)
%E More terms from _Michel Marcus_, Jun 12 2013
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