cp's OEIS Frontend

A328638 a(n) is the sum of m such that 2 <= m <= n and gpf(m) = gpf(n), where gpf(i) is the greatest prime factor of i (A006530), with a(1) = 1.

%I A328638 #30 Dec 05 2019 20:02:29
%S A328638 1,2,3,6,5,9,7,14,18,15,11,30,13,21,30,30,17,48,19,50,42,33,23,72,75,
%T A328638 39,99,70,29,105,31,62,66,51,105,135,37,57,78,145,41,147,43,110,190,
%U A328638 69,47,183,196,240,102,130,53,237,165,252,114,87,59,300
%N A328638 a(n) is the sum of m such that 2 <= m <= n and gpf(m) = gpf(n), where gpf(i) is the greatest prime factor of i (A006530), with a(1) = 1.
%C A328638 For n >= 2, a(n) is the sum of the terms that precede n on the row, of the A125624 array, that contains n.
%e A328638 5, 10, 15 and 20 have same gpf as 20, so a(20) = 5 + 10 + 15 + 20 = 50.
%o A328638 (PARI)
%o A328638 gpf(n)={my(f=factor(n)[,1]); f[#f]}
%o A328638 a(n)={if(n<=1, n==1, my(t=gpf(n)); sum(i=2, n, if(gpf(i)==t, i, 0)))} \\ _Andrew Howroyd_, Oct 28 2019
%Y A328638 Cf. A006530 (gpf), A125624, A078899, A328637.
%K A328638 nonn,easy
%O A328638 1,2
%A A328638 _J. Stauduhar_, Oct 22 2019