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 A210257 #35 Jan 11 2020 21:13:04 %S A210257 1,212,32423,4364634,548696845,651081291281056,7612101512161215101267, %T A210257 87141218152016201518121478,981614211824202520241821141689, %U A210257 109181624212824302530242821241618910 %N A210257 Construct a triangle with centered rows in which row n contains n copies of n; form the componentwise sum of the first n rows and concatenate these decimal numbers to get a(n). %C A210257 A109242 may be constructed using a similar pyramid algorithm: %C A210257 ....1..........a(1) = 1 %C A210257 ...1 1.........a(2) = 111 %C A210257 ..1 1 1........a(3) = 11211 %C A210257 .1 1 1 1.......a(4) = 1122211 %e A210257 ...1................a(1) = 1 %e A210257 ..2 2...............a(2) = 212 %e A210257 .3 3 3..............a(3) = 32423 %e A210257 4 4 4 4.............a(4) = 4364634 %K A210257 nonn,base %O A210257 1,2 %A A210257 _Dave Durgin_, Mar 19 2012 %E A210257 Definition edited by _N. J. A. Sloane_, Jan 11 2020