A068821 Triangular numbers such that both the number formed by the first floor((k+1)/2) digits and the remaining part (if any) are triangular numbers, where k is the number of digits.

1, 3, 6, 10, 36, 66, 136, 300, 378, 666, 2145, 3655, 10153, 21528, 66066, 78210, 91378, 190036, 210276, 378015, 406351, 630003, 666435, 741153, 946000, 990528, 1537381, 1710325, 3788128, 4652775, 9037626, 87787875

Offset

1,2

Links

Robert Israel, Table of n, a(n) for n = 1..100

Example

210276 is a triangular number with k=6 digits and the numbers 210 and 276 are both triangular numbers.

Maple

filter:= proc(n) local k;
    k:= ceil((1+ilog10(n))/2);
    issqr(1+8*floor(n/10^k)) and issqr(1+8*(n mod 10^k))
end proc:
select(filter, [seq(i*(i+1)/2, i=1..20000)]); # Robert Israel, Aug 18 2019

Crossrefs

Sequence in context: A151376 A066245 A356977 * A062100 A262242 A339783

Adjacent sequences: A068818 A068819 A068820 * A068822 A068823 A068824

Keyword

nonn,base

Author

Amarnath Murthy, Mar 08 2002

Extensions

More terms from Sascha Kurz, Mar 17 2002

Status

approved