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 A072521 a(1) = 6 and then the smallest triangular numbers such that sum of two neighbors is also a triangular number. 1
 6, 15, 21, 45, 91, 990, 1711, 365085, 401856, 713415, 785631, 1079715, 1326006, 2355535, 2888406, 5137615, 5666661, 5764710, 9550635, 9921285, 10934826, 19434495, 21421785, 23622501, 42003195, 46315500, 82349361, 146384605 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The sequence is unbounded as a(n+1) is less than or equal to the n-th triangular number. LINKS EXAMPLE 45 is a term because 21 + 45 = 66, 45 + 91 = 136, and 66 and 136 are triangular numbers. PROG (PARI) p=6; k=3; print1(p", "); for(n=1, 30, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u)); while(u!=t*(t+1)/2, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u))); p=k*(k+1)/2; print1(p", ")) CROSSREFS Cf. A072522. Sequence in context: A063466 A138109 A332877 * A130178 A100410 A095032 Adjacent sequences:  A072518 A072519 A072520 * A072522 A072523 A072524 KEYWORD nonn AUTHOR Amarnath Murthy, Jul 31 2002 EXTENSIONS More terms from Ralf Stephan, Mar 30 2003 STATUS approved

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Last modified August 12 21:10 EDT 2020. Contains 336440 sequences. (Running on oeis4.)