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%I #25 Dec 07 2019 12:18:23
%S 1,121,169,841,2047288797225,61838862885361
%N Squares whose sum of anti-divisors is a square.
%C See A066272 for definition of anti-divisor.
%C a(5), if it exists, is greater than 10^12. - _Franklin T. Adams-Watters_, Feb 08 2012
%C a(7) > 10^16. - _Hiroaki Yamanouchi_, Sep 27 2015
%o (Python)
%o from gmpy2 import is_square
%o from sympy import divisors
%o A074730 = [n for n in (x**2 for x in range(1,10**4)) if is_square(int(sum([2*d for d in divisors(n) if n > 2*d and n%(2*d)] + [d for d in divisors(2*n-1) if n > d >=2 and n%d] + [d for d in divisors(2*n+1) if n > d >=2 and n%d])))]
%o # _Chai Wah Wu_, Aug 12 2014
%o (PARI) has(n)=my(k=valuation(n, 2)); issquare(sigma(2*n+1)+sigma(2*n-1)+sigma(n>>k)<<(k+1)-6*n-2)
%o print1(1); for(n=2,1e6, if(has(n^2), print1(", "n^2))) \\ _Charles R Greathouse IV_, Nov 20 2015
%Y Cf. A066417.
%K nonn
%O 1,2
%A _Jason Earls_, Sep 05 2002
%E a(5)-a(6) from _Hiroaki Yamanouchi_, Sep 27 2015
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