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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 (list; graph; refs; listen; history; text; internal format)
 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 Adjacent sequences: A069699 A069700 A069701 * A069703 A069704 A069705 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

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Last modified April 2 02:09 EDT 2023. Contains 361723 sequences. (Running on oeis4.)