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 A169836 Perfect squares that are a product of two distinct triangular numbers. 2
 0, 36, 900, 1225, 7056, 32400, 41616, 44100, 88209, 108900, 298116, 705600, 1368900, 1413721, 1498176, 2924100, 5336100, 8643600, 8820900, 9217296, 10432900, 15210000, 24147396, 37088100, 48024900, 50893956, 50979600, 52490025, 55353600, 80568576 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(47) = 1728896400 is the product of two distinct triangular numbers in two different ways. 1728896400 = A000217(8) * A000217(9800) = A000217(27) * A000217(3024). - Donovan Johnson, Sep 01 2012 LINKS Donovan Johnson, Table of n, a(n) for n = 1..1000 Erich Friedman, What's Special About This Number? (See entry 7056.) FORMULA a(n) = (A175497(n))^2. [From R. J. Mathar, Jun 03 2010] EXAMPLE Examples: 900=3*300. 7056 = 6*1176. 1368900 = 6*228150. 44100 = 36*1225. PROG (PARI) istriangular(n)=issquare(8*n+1) isok(n) = {if (issquare(n), d = divisors(n); fordiv(n, d, if (d > sqrtint(n), break); if ((d != n/d) && istriangular(d) && istriangular(n/d), return (1)); ); ); return (0); } \\ Michel Marcus, Jul 24 2013 CROSSREFS Cf. A000217, A054731, A169835. Sequence in context: A166790 A001812 A229680 * A248108 A233003 A075916 Adjacent sequences:  A169833 A169834 A169835 * A169837 A169838 A169839 KEYWORD nonn AUTHOR R. J. Mathar, May 30 2010 EXTENSIONS More terms from R. J. Mathar, Jun 03 2010 STATUS approved

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Last modified January 20 07:59 EST 2020. Contains 331081 sequences. (Running on oeis4.)