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 A372808 #19 May 18 2024 19:35:13
%S A372808 1,0,2,3,3,6,4,8,10,11,10,18,18,13,9,14,18,26,24,29,26,27,27,29,32,37,
%T A372808 34,34,40,38,36,39,46,49,38,47,39,49,44,54,60,57,60,64,66,71,52,48,55,
%U A372808 63,71,67,70,59,52,52,71,85,96,96,84,89,87,85,76,74,71,80
%N A372808 a(n) = sum of the digits (mod 5) of 5^n.
%H A372808 Paolo Xausa, <a href="/A372808/b372808.txt">Table of n, a(n) for n = 0..10000</a>
%H A372808 J. M. Borwein and P. B. Borwein, <a href="https://doi.org/10.2307/2324993">Strange Series and High Precision Fraud</a>, The American Mathematical Monthly, Vol. 99, No. 7 (1992), pp. 622-640.
%F A372808 Sum_{n >= 1} a(n)/5^n = 1/9. See Example 5.1 (e) in Borwein and Borwein (1992), p. 639.
%e A372808 a(7) = 8 since 5^7 = 78125 and (7 mod 5) + (8 mod 5) + (1 mod 5) + (2 mod 5) + (5 mod 5) = 2 + 3 + 1 + 2 + 0 = 8.
%t A372808 Array[Total[Mod[IntegerDigits[5^#], 5]] &, 100, 0]
%o A372808 (PARI) a(n) = my(d=digits(5^n)); vecsum(apply(x->(x % 5), d)); \\ _Michel Marcus_, May 17 2024
%Y A372808 Cf. A000351, A010874, A066001.
%K A372808 nonn,base,easy
%O A372808 0,3
%A A372808 _Paolo Xausa_, May 13 2024