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A069702
Triangular numbers with internal digits also forming a triangular number.
2
105, 136, 210, 231, 300, 406, 435, 465, 561, 630, 666, 703, 861, 903, 1035, 2016, 2211, 2556, 3003, 3916, 4005, 5151, 6105, 6216, 6555, 6786, 8001, 8911, 9453, 10011, 10153, 19900, 20100, 20910, 23005, 28203, 39903, 41905, 46665, 48205
OFFSET
1,1
COMMENTS
Includes (2*10^(2*k)-10^k-1)/9 for every k, so the sequence is infinite. - Robert Israel, Aug 28 2018
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..1000 (first 210 terms from Robert Israel)
EXAMPLE
3916 is a member as 3916 and 91 both are triangular.
MAPLE
filter:= proc(x) local t, m;
t:= floor(x/10);
m:= ilog10(t);
t:= t - floor(t/10^m)*10^m;
issqr(8*t+1)
end proc:
select(filter, [seq(x*(x+1)/2, x=14..1000)]); # Robert Israel, Aug 28 2018
MATHEMATICA
s=Select[Range[14, 400], IntegerQ[Sqrt[1+8FromDigits[Drop[Drop[IntegerDigits[ #(#+1)/2], 1], -1]]]]&]; s(s+1)/2
CROSSREFS
Cf. A077366.
Sequence in context: A267891 A097217 A115935 * A239589 A203614 A252069
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Apr 06 2002
EXTENSIONS
Edited by Dean Hickerson, Apr 09 2002
Edited by Robert Israel, Aug 28 2018
STATUS
approved