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A125597
a(1)=1; a(n) = Sum_{1<=k<n, gcd(k,n(n+1)/2)=1} a(k).
2
1, 1, 1, 2, 4, 8, 6, 11, 21, 51, 11, 22, 133, 159, 151, 328, 707, 1414, 880, 1732, 3850, 9482, 1742, 3480, 22126, 37243, 25604, 51381, 102087, 204174, 157324, 285010, 660221, 1285026, 262885, 547906, 3664304, 5844380, 3927062, 8543954, 19956539
OFFSET
1,4
LINKS
EXAMPLE
The positive integers < 8 and coprime to 8*9/2 = 36 are 1,5,7. So a(8) = a(1)+a(5)+a(7) = 1+4+6 = 11.
MAPLE
A[1]:= 1:
for n from 2 to 100 do
A[n]:= add(A[j], j=select(k -> igcd(k, n*(n+1)/2)=1, [$1..n-1]))
od:
seq(A[i], i=1..100); # Robert Israel, Mar 28 2018
MATHEMATICA
f[l_List] := Block[{n = Length[l] + 1}, Append[l, Plus @@ l[[Select[Range[n - 1], GCD[ #, n*(n + 1)/2] == 1 &]]]]]; Nest[f, {1}, 40] (* Ray Chandler, Nov 26 2006 *)
CROSSREFS
Cf. A125596.
Sequence in context: A121711 A249681 A256561 * A366170 A151732 A109554
KEYWORD
nonn
AUTHOR
Leroy Quet, Nov 26 2006
EXTENSIONS
Extended by Ray Chandler, Nov 26 2006
STATUS
approved