A078795 - OEIS

%I #9 Jan 13 2021 21:27:47

%S 1,13,136,13610,1361015,136101521,13610152128,1361015212836,

%T 136101521283645,13610152128364555,1361015212836455566,

%U 136101521283645556678,13610152128364555667891,13610152128364555667891105,13610152128364555667891105120

%N Concatenate first n triangular numbers.

%C There are only 2 primes i.e. 13 and 136101521 known in the sequence for first 1000 terms of the sequence.

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

%e a(3)=136 because 1, 3, 6 are first 3 triangular numbers.

%p a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(a(n-1), n*(n+1)/2))) 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, 1, m}]]]], {m, 1, 20}]

%Y Cf. A000217.

%K base,easy,nonn

%O 1,2

%A _Shyam Sunder Gupta_, Jan 10 2003