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A060658 Even numbers n such that sigma(x) = n has no solution.

%I A060658 #13 Oct 01 2018 03:10:08
%S A060658 2,10,16,22,26,34,46,50,52,58,64,66,70,76,82,86,88,92,94,100,106,116,
%T A060658 118,122,130,134,136,142,146,148,154,166,170,172,178,184,188,190,196,
%U A060658 202,206,208,214,218,220,226,232,236,238,244,246,250,254,262,268,274
%N A060658 Even numbers n such that sigma(x) = n has no solution.
%C A060658 Contains 2*p if p is a prime not in A005382. - _Robert Israel_, Sep 30 2018
%H A060658 Robert Israel, <a href="/A060658/b060658.txt">Table of n, a(n) for n = 1..10000</a>
%e A060658 a(2) = 10 because there is no x < 10 whose sigma(x) = 10, an even number.
%p A060658 N:= 1000: # to get all terms <= N
%p A060658 V:= Vector(N):
%p A060658 for x from 1 to N-1 do
%p A060658   v:= numtheory:-sigma(x);
%p A060658   if v <= N then V[v]:= 1 fi
%p A060658 od:
%p A060658 select(t -> V[t]=0, [seq(i,i=2..N,2)]); # _Robert Israel_, Sep 30 2018
%t A060658 a = {}; Do[ s = DivisorSigma[ 1, n ]; If[ EvenQ[ s ], a = Append[ a, s ] ], {n, 1, 400} ]; Complement[ Table[ 2n, {n, 1, 200} ], Union[ a ] ]
%o A060658 (PARI) isok(n) = !(n%2) && !sum(k=1, n, sigma(k)==n); \\ _Michel Marcus_, Mar 08 2017
%Y A060658 Cf. A005382, A007369.
%K A060658 nonn
%O A060658 1,1
%A A060658 _Robert G. Wilson v_, Apr 18 2001