OFFSET
1,1
COMMENTS
Contains 2*p if p is a prime not in A005382. - Robert Israel, Sep 30 2018
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(2) = 10 because there is no x < 10 whose sigma(x) = 10, an even number.
MAPLE
N:= 1000: # to get all terms <= N
V:= Vector(N):
for x from 1 to N-1 do
v:= numtheory:-sigma(x);
if v <= N then V[v]:= 1 fi
od:
select(t -> V[t]=0, [seq(i, i=2..N, 2)]); # Robert Israel, Sep 30 2018
MATHEMATICA
a = {}; Do[ s = DivisorSigma[ 1, n ]; If[ EvenQ[ s ], a = Append[ a, s ] ], {n, 1, 400} ]; Complement[ Table[ 2n, {n, 1, 200} ], Union[ a ] ]
PROG
(PARI) isok(n) = !(n%2) && !sum(k=1, n, sigma(k)==n); \\ Michel Marcus, Mar 08 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Apr 18 2001
STATUS
approved