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 A057246 s=0; d is divisor of n [here d<= n/d)]; if gcd(d,n/d)=1 or gcd(d,n/d)=d then s=s+d+(n/d); [if d=n/d then s=s+d]: The sequence contains composite n for which s = 2*n. 0
 6, 28, 6966, 15066 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS s(n) is a vaguely unitary analog of A060866. - R. J. Mathar, Oct 24 2011 No more terms up to 10^9. - Michel Marcus, Feb 24 2016 LINKS EXAMPLE a(2)=28, gcd(1,28)=gcd(4,28/4)=1, gcd(2,28/2)=2, 1+28+4+7+2+14=56. 56-28=28. MATHEMATICA f[ n_Integer ] := (ds = Divisors[ n ]; sq = N[ Sqrt[ n ] ]; l = 1; While[ ds[[ l ] ] <= sq, l++ ]; l = l - 1; ds = Take[ ds, l ]; s = 1; k = 2; While[ k <= l, If[ GCD[ ds[[ k ] ], n/ds[[ k ] ] ] == 1 || GCD[ ds[[ k ] ], n/ds[[ k ] ] ] == ds[[ k ] ], s = s + ds[[ k ] ] + n/ds[[ k ] ] ]; k++ ]; If[ ds[[ -1 ] ] == n/ds[[ -1 ] ], s = s - d ]; s) Do[ If[ ! PrimeQ[ n ] && f[ n ] == n, Print[ n ] ], {n, 2, 33429000} ] (* Robert G. Wilson v, Nov 09 2000 *) PROG (PARI) is(n)=my(s, g); fordiv(n, d, if(d^2

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Last modified October 15 12:31 EDT 2019. Contains 328026 sequences. (Running on oeis4.)