login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

a(n) = the smallest number m such that gcd(sigma(m), pod(m)) = n where sigma(k) = the sum of the divisors of k (A000203) and pod(k) = the product of the divisors of k (A007955).
1

%I #7 Sep 08 2022 08:46:24

%S 1,10,15,12,95,180,91,56,51,40,473,6,117,980,135,70,1139,90,703,290,

%T 861,26378,3151,54,745,468,255,2156,5017,26100,775,124,1419,2176,4865,

%U 96,2701,26714,585,190,6683,65268,11051,5632,435,144946,13207,42,679,5800

%N a(n) = the smallest number m such that gcd(sigma(m), pod(m)) = n where sigma(k) = the sum of the divisors of k (A000203) and pod(k) = the product of the divisors of k (A007955).

%C a(n) = the smallest number m such that A306682(m) = n.

%e For n=2; a(2) = 10 because gcd(sigma(10), pod(10)) = gcd (18, 100) = 2 and 10 is the smallest.

%o (Magma) [Min([n: n in[1..10^5] | GCD(SumOfDivisors(n), &*[d: d in Divisors(n)]) eq k]): k in [1..45]]

%o (PARI) f(n) = my(d=divisors(n)); gcd(vecsum(d), vecprod(d)); \\ A306682

%o a(n) = {my(k=1); while (f(k) != n, k++); k;} \\ _Michel Marcus_, Mar 05 2019

%Y Cf. A000005, A000203, A007955, A074391, A306682.

%K nonn

%O 1,2

%A _Jaroslav Krizek_, Mar 05 2019