login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Greatest common divisors of consecutive terms of numbers of the form 3^i*5^j.
2

%I #8 Feb 06 2020 07:27:44

%S 1,1,1,3,5,1,9,15,3,1,5,45,9,3,15,5,25,27,9,45,15,75,81,1,125,135,45,

%T 225,243,3,375,405,5,625,675,729,9,1125,1215,15,1875,2025,2187,1,3125,

%U 3375,3645,45,5625,6075,6561,3,9375,10125,10935,5,15625,16875,18225

%N Greatest common divisors of consecutive terms of numbers of the form 3^i*5^j.

%H Amiram Eldar, <a href="/A112752/b112752.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = gcd(A003593(n), A003593(n+1)).

%t s = {}; m = 12; Do[n = 5^k; While[n <= 5^m, AppendTo[s, n]; n *= 3], {k, 0, m}]; Map[GCD @@ # &, Partition[Union[s], 2, 1]] (* _Amiram Eldar_, Feb 06 2020 *)

%Y Cf. A003593, A112757.

%K nonn

%O 1,4

%A _Reinhard Zumkeller_, Sep 18 2005