This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A121007 #13 Sep 15 2024 02:47:12 %S A121007 1,5,25,25,625,3125,15625,78125,78125,1953125,9765625,48828125, %T A121007 244140625,244140625,1220703125,6103515625,30517578125,152587890625, %U A121007 30517578125,3814697265625,19073486328125 %N A121007 Denominators of partial alternating sums of Catalan numbers scaled by powers of 1/5. %C A121007 Numerators are given under A121006. See A121006 for further comments. %F A121007 a(n)=denominator(r(n)) with r(n) := rIII(p=1,n) = sum(((-1)^k)*C(k)/5^k,k=0..n) and C(k):=A000108(k) (Catalan). The rationals r(n) are given in lowest terms. %e A121007 Rationals r(n): 1, 4/5, 22/25, 21/25, 539/625, 2653/3125, 13397/15625, 66556/78125, 66842/78125, 1666188/1953125, 8347736/9765625,... %Y A121007 The second member (p=2) is A121008/A121009. %K A121007 nonn,frac,easy %O A121007 0,2 %A A121007 _Wolfdieter Lang_, Aug 16 2006