cp's OEIS Frontend

A178380 The greatest common prime divisor of n, A000032(n)-1 and A001608(n), or 1 if no such greatest common divisor exists.

%I A178380 #7 Oct 23 2015 02:34:29
%S A178380 2,3,2,5,1,7,2,3,1,11,1,13,1,1,2,17,1,19,1,1,2,23,1,5,1,3,1,29,1,31,2,
%T A178380 1,1,1,1,37,1,1,1,41,1,43,2,1,2,47,1,7,2,1,1,53,1,1,1,1,2,59,1,61,1,1,
%U A178380 2,1,1,67,1,1,1,71,1,73,2,1,1,1,1,79,1,3,1,83,1,1,2,1,2,89,1
%N A178380 The greatest common prime divisor of n, A000032(n)-1 and A001608(n), or 1 if no such greatest common divisor exists.
%C A178380 Records of the sequence are consecutive primes.
%p A178380 A000032 := proc(n) ((1+sqrt(5))/2)^n + ((1-sqrt(5))/2)^n ; expand(%) ; end proc:
%p A178380 A001608 := proc(n) coeftayl( (3-x^2)/(1-x^2-x^3),x=0,n) ; end proc:
%p A178380 A178380 := proc(n) local g; g := igcd(n, A000032(n)-1, A001608(n)) ; if g = 1 then 1; else numtheory[factorset](%) ; max( op(%)) ; end if; end proc:
%p A178380 seq(A178380(n),n=2..90) ; # _R. J. Mathar_, Aug 08 2010
%Y A178380 Cf. A000032, A001608, A178375.
%K A178380 nonn
%O A178380 2,1
%A A178380 _Vladimir Shevelev_, May 26 2010
%E A178380 More terms from _R. J. Mathar_, Aug 08 2010