A078891 - OEIS

%I #10 Jan 13 2021 21:27:29

%S 1,31,631,10631,1510631,211510631,28211510631,3628211510631,

%T 453628211510631,55453628211510631,6655453628211510631,

%U 786655453628211510631,91786655453628211510631,10591786655453628211510631,12010591786655453628211510631

%N Concatenate first n triangular numbers in reverse order.

%D Shyam Sunder Gupta, Smarandache Sequence of Triangular Numbers, Smarandache Notions Journal, (to appear in Vol. 14, 2003).

%e a(3)=631 because 6,3,1 are first 3 triangular numbers in reverse order.

%p a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(n*(n+1)/2, a(n-1)))) end:

%p seq(a(n), n=1..20);  # _Alois P. Heinz_, Jan 13 2021

%t Table[FromDigits[Flatten[IntegerDigits[Table[n*(n+1)/2, {n, m, 1, -1}]]]], {m, 1, 20}]

%Y Cf. A000217.

%K base,easy,nonn,less

%O 1,2

%A _Shyam Sunder Gupta_, Jan 10 2003