OFFSET
1,1
COMMENTS
Or, Chowla's function of n is a positive square.
LINKS
Zak Seidov, Table of n, a(n) for n = 1..1000
EXAMPLE
MAPLE
select(t -> not isprime(t) and issqr(numtheory:-sigma(t)-1-t), [$2..2000]); # Robert Israel, Oct 25 2017
MATHEMATICA
Select[Range@ 1500, And[! PrimeQ@ #, IntegerQ@ Sqrt[DivisorSigma[1, #] - # - 1]] &] (* Michael De Vlieger, Oct 25 2017 *)
PROG
(PARI) {for(n=1, 2000, spf=sumdiv(n, x, x)-1-n; if(spf>0&&issquare(spf), print1(n", ")))}
(Magma) IsA187086:=func< n | not IsPrime(n) and IsSquare(&+[ a: a in Divisors(n) | a ne 1 and a ne n ]) >; [ n: n in [2..2000] | IsA187086(n) ]; // Klaus Brockhaus, Mar 04 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Mar 04 2011
STATUS
approved