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A105289
Indices of Lucas 5-step numbers A074048 which have a nontrivial divisor in common with index.
2
6, 12, 18, 20, 21, 33, 36, 48, 51, 54, 55, 60, 75, 78, 87, 99, 100, 108, 110, 112, 114, 120, 129, 132, 133, 144, 147, 153, 154, 155, 159, 162, 165, 174, 177, 180, 182, 183, 185, 195, 210, 219, 225, 228, 230, 234, 237, 245, 261, 267, 270, 275, 285, 290, 297, 310
OFFSET
1,1
COMMENTS
Extension by T. D. Noe. Wanted: closed-form formula for this as exists for Fibonacci and Lucas numbers. See also A105764 (indices of prime Lucas 5-step numbers).
LINKS
FORMULA
gcd(a(n), A074048(a(n))) > 1.
EXAMPLE
gcd(6, A074048(6)) = gcd(6,57) = 3,
gcd(20, A074048(20)) = gcd(20,743775) = 5.
gcd(21, A074048(21)) = gcd(21,1462223) = 7.
MATHEMATICA
m=300; s = LinearRecurrence[{1, 1, 1, 1, 1}, {5, 1, 3, 7, 15}, m+1]; Select[Range[m], !CoprimeQ[#, s[[#+1]]] &] (* Amiram Eldar, Sep 05 2019 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Apr 25 2005
EXTENSIONS
More terms from Amiram Eldar, Sep 05 2019
STATUS
approved