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A127942
a(n) = denominator of b(n), where b(1)=1, b(n) = Sum_{1<=k<n, gcd(k,n)=1} 1/b(k).
1
1, 1, 1, 2, 6, 19, 2850, 459458, 216537731091, 4850944054979611, 7043380548155783510819615297769488951475, 9278148088243438548919355731906562181020842484
OFFSET
1,4
EXAMPLE
{b(n)}: 1, 1, 2, 3/2, 19/6, 25/19, 12091/2850, ... Since 1 and 5 are the positive integers which are coprime to 6 and are < 6, b(6) = 1/b(1) + 1/b(5) = 1 + 6/19 = 25/19.
MATHEMATICA
f[l_List] := Block[{n = Length[l] + 1, d}, d = Select[Range[n - 1], GCD[ #, n] == 1 &]; Append[l, Sum[1/l[[d[[i]]]], {i, Length[d]}]]]; Denominator[Nest[f, {1}, 12]] (* Ray Chandler, Feb 09 2007 *)
CROSSREFS
Cf. A127941.
Sequence in context: A341639 A186770 A332406 * A110956 A298446 A364563
KEYWORD
frac,nonn
AUTHOR
Leroy Quet, Feb 08 2007
EXTENSIONS
Extended by Ray Chandler, Feb 09 2007
STATUS
approved