OFFSET
1,6
COMMENTS
a(n) = 1 if and only if n is prime. - Robert Israel, Sep 28 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
f:= proc(n) local v;
v:= numtheory:-sigma(n)-numtheory:-phi(n);
if v::even then v/2 else -1 fi
end proc:
map(f, [$1..100]); # Robert Israel, Sep 28 2017
CROSSREFS
KEYWORD
sign
AUTHOR
N. J. A. Sloane, Sep 28 2017
STATUS
approved