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A109321
Admirable numbers n such that the subtracted divisor is > sqrt(n).
2
24, 30, 84, 120, 140, 224, 234, 270, 308, 364, 476, 532, 644, 672, 1488, 1638, 2480, 3472, 3724, 4095, 5456, 5624, 6200, 6435, 6448, 8432, 9424, 11408, 14384, 15872, 18352, 20336, 21328, 23312, 24384, 26288, 29264, 29450, 30256, 33232, 35150
OFFSET
1,1
COMMENTS
Solutions to sigma(x)=2(x+d) with d > sqrt(x) being a proper divisor of x. The subsequence A165772 contains most of the terms. - M. F. Hasler, Oct 11 2009
LINKS
EXAMPLE
a(2)=30 because 1+2+3+5+10+15-6 = 30 and 6 > sqrt(30) = 5.477...
MATHEMATICA
aQ[n_] := (d = DivisorSigma[1, n] - 2n) > 0 && EvenQ[d] && Mod[n, d/2] == 0 && d < 2n && d^2 > 4n; Select[Range[35150], aQ] (* Amiram Eldar, Sep 22 2019 *)
PROG
(PARI) is_A109321(n)= my(d=sigma(n)-2*n); d>0 && bittest(d, 0)==0 && d<2*n && d*d>4*n && 2*n%d==0 \\ M. F. Hasler, Oct 11 2009
CROSSREFS
Cf. A111592.
Sequence in context: A372991 A333122 A135045 * A349689 A348630 A067952
KEYWORD
nonn
AUTHOR
Jason Earls, Aug 20 2005
STATUS
approved