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A060658
Even numbers n such that sigma(x) = n has no solution.
2
2, 10, 16, 22, 26, 34, 46, 50, 52, 58, 64, 66, 70, 76, 82, 86, 88, 92, 94, 100, 106, 116, 118, 122, 130, 134, 136, 142, 146, 148, 154, 166, 170, 172, 178, 184, 188, 190, 196, 202, 206, 208, 214, 218, 220, 226, 232, 236, 238, 244, 246, 250, 254, 262, 268, 274
OFFSET
1,1
COMMENTS
Contains 2*p if p is a prime not in A005382. - Robert Israel, Sep 30 2018
LINKS
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
Sequence in context: A138632 A175957 A307055 * A239321 A054028 A063716
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Apr 18 2001
STATUS
approved