Search: id:A067392
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%I A067392
%S A067392 0,2,3,6,5,15,7,20,18,35,11,54,13,63,60,72,17,117,19,130,105,143,23,
%T A067392 204,75,195,135,238,29,345,31,272,231,323,210,450,37,399,312,500,41,
%U A067392 651,43,550,495,575,47,792,196,775,510,754,53,999,440,924,627,899,59
%N A067392 Sum of numbers <= n which have common factors with n.
%C A067392 Sum of k <= n such that GCD(n,k) > 1.
%F A067392 For n>=2, a(n)=n(n+1)/2 - n phi(n)/2.
%F A067392 Not multiplicative.
%F A067392 a(p) = p where p is a prime; a(2^k) = 2^(k-1)*{2^(k-1) + 1).
%e A067392 n=24, a(n)=2+3+4+6+8+9+10+12+14+15+16+18+20+21+22+24=204.
%t A067392 a[n_] := Plus@@Select[Range[1, n], GCD[ #, n]>1&]
%t A067392 f[n_] := If[PrimeQ[n], n, k = Log[2, n]; If[ IntegerQ[k], 2^(k - 1)(2^(k 
               - 1) + 1), (n/2)(n + 1 - EulerPhi[n])]]; Table[ f[n], {n, 2, 60}] 
               (from RWGv)
%Y A067392 Cf. A000203, A000217, A023896, A024816.
%Y A067392 Sequence in context: A136183 A100211 A071257 this_sequence A066449 A098012 
               A066117
%Y A067392 Adjacent sequences: A067389 A067390 A067391 this_sequence A067393 A067394 
               A067395
%K A067392 nonn
%O A067392 1,2
%A A067392 Labos E. (labos(AT)ana.sote.hu), Jan 22 2002

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