A284229 - OEIS

%I #29 Mar 28 2017 09:54:03

%S 1,10,12,6,336,24,5952,168,792,496,666624,270,10924032,6720,7344,120,

%T 3757637632,4284,45091651584,2160,79488,1820672,11544784011264,672,

%U 298080,29331456,106200,13440,53620880789471232,10080,1501384662105194496,6552,7022592,7515275264

%N a(n) is the least k such that A073802(k) = n.

%C Composite numbers that have just 1 as divisor that satisfies the condition for which sigma(k) / d_i is an integer are the Duffinian numbers (A003624).

%C Alternative definition: Least k such that d(gcd(k,sigma(k)) = n. - _Giovanni Resta_, Mar 23 2017

%e The divisors of 12 are 1, 2, 3, 4, 6, 12 and sigma(12) = 28. Then:

%e 1) 28 / 1 = 28;

%e 2) 28 / 2 = 14;

%e 3) 28 / 4 = 7;

%e and 12 is the least number to have this property. Therefore a(3) = 12.

%p with(numtheory): P:=proc(q) local k,n; for k from 1 to q do

%p for n from 1 to q do if tau(gcd(n,sigma(n)))=k then

%p print(n); break; fi; od; od; end: P(10^9);

%t TakeWhile[#, # > 0 &] &@ Table[If[KeyExistsQ[#, n], First@ Lookup[#, n], -1], {n, Max@ Keys@ #}] &@ KeySort@ PositionIndex@ Table[DivisorSum[k, 1 &, IntegerQ[DivisorSigma[1, k]/#] &], {k, 10^6}] (* per Name, Version 10, or *)

%t TakeWhile[#, # > 0 &] &@ Table[If[KeyExistsQ[#, n], First@ Lookup[#, n], -1], {n, Max@ Keys@ #}] &@ KeySort@ PositionIndex@ Table[DivisorSigma[0, GCD[k, DivisorSigma[1, k]]], {k, 10^7}] (* faster, Version 10, _Michael De Vlieger_, Mar 24 2017 *)

%o (PARI) nb(n) = my(s = sigma(n)); sumdiv(n, d, (s % d) == 0);

%o a(n) = k=1; while(nb(k) != n, k++); k; \\ _Michel Marcus_, Mar 24 2017

%Y Cf. A000203, A003624, A073802.

%K nonn

%O 1,2

%A _Paolo P. Lava_, Mar 23 2017

%E a(13), a(17), a(19) and from a(22) to a(34) from _Giovanni Resta_, Mar 23 2017

%E Name proposed by _Michel Marcus_, Mar 24 2017