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 A286917 Numbers k such that there is an anti-divisor d of k satisfying sigma(d) = k. 0
 3, 4, 13, 32, 40, 60, 121, 364, 1093, 3200, 3280, 9841, 15120, 16380, 29282, 29524, 88573, 91728, 264992, 265720, 797161 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE Anti-divisors of 60 are 7, 8, 11, 17, 24, 40 and sigma(24) = 60. MAPLE with(numtheory): P:= proc(q) local a, k, n; for n from 3 to q do a:=[]; for k from 2 to n-1 do if abs((n mod k)-k/2)<1 then a:=[op(a), k]; fi; od; for k from 1 to nops(a) do if n=sigma(a[k]) then print(n); break; fi; od; od; end: P(10^4); # Paolo P. Lava, May 16 2017 PROG (PARI) isok(n) = {ad = select(t->n%t && t

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Last modified September 15 18:22 EDT 2019. Contains 327082 sequences. (Running on oeis4.)