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 A086989 Sum of even-indexed divisors of n. 1

%I #9 May 19 2019 11:36:51

%S 0,2,3,2,5,8,7,10,3,12,11,18,13,16,18,10,17,26,19,27,24,24,23,38,5,28,

%T 30,37,29,47,31,42,36,36,40,33,37,40,42,57,41,64,43,57,57,48,47,78,7,

%U 62,54,67,53,80,60,79,60,60,59,104,61,64,75,42,70,96,67,87,72,93,71,122,73

%N Sum of even-indexed divisors of n.

%C a(p)=p iff p prime.

%e 10 has divisors 1,2,5 and 10. The even indices yield 2 and 10, so a(10)=12.

%t Prepend[Table[Total[Transpose[Partition[Divisors[n],2]][[2]]],{n,2,100}],0] (* _Harvey P. Dale_, Jan 17 2011 *)

%o (PARI) for (n=1,100,s=0; c=0; fordiv(i=n,i,c++; if (c%2==0,s+=i)); print1(","s))

%Y Cf. A086988.

%K nonn

%O 1,2

%A _Jon Perry_, Jul 27 2003

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Last modified November 29 20:40 EST 2023. Contains 367447 sequences. (Running on oeis4.)