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 A053837 #21 Jan 16 2025 16:11:07
%S A053837 0,1,2,3,4,5,6,7,8,9,1,2,3,4,5,6,7,8,9,0,2,3,4,5,6,7,8,9,0,1,3,4,5,6,
%T A053837 7,8,9,0,1,2,4,5,6,7,8,9,0,1,2,3,5,6,7,8,9,0,1,2,3,4,6,7,8,9,0,1,2,3,
%U A053837 4,5,7,8,9,0,1,2,3,4,5,6,8,9,0,1,2,3,4,5,6,7,9,0,1,2,3,4,5,6,7,8,1,2,3,4,5
%N A053837 Sum of digits of n modulo 10.
%F A053837 a(n) = A010879(A007953(n)) = (n+a(floor[n/10])) mod 10. So can construct sequence by starting with 0 and mapping 0->0123456789, 1->1234567890, 2->2345678901 etc. (e.g. 0, 0123456789, 0123456789123456789023456789013456789012456..., etc.) and looking at n-th digit of a term with sufficient digits.
%e A053837 a(59)=4 because 5+9 = 14 = 4 mod 10.
%t A053837 Table[Mod[Total[IntegerDigits[n]],10],{n,0,120}] (* _Harvey P. Dale_, Oct 02 2018 *)
%o A053837 (PARI) a(n)=sumdigits(n)%10 \\ _Charles R Greathouse IV_, Feb 02 2023
%Y A053837 Cf. A007953, A010879.
%Y A053837 Cf. A000120, A010060, A053838, A053839, A053840, A053841, A053842, A053843, A053844.
%K A053837 base,nonn,easy
%O A053837 0,3
%A A053837 _Henry Bottomley_, Mar 28 2000