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 A072502 Numbers that are run sums (trapezoidal, the difference between two triangular numbers) in exactly 3 ways. 11
 9, 18, 25, 36, 49, 50, 72, 98, 100, 121, 144, 169, 196, 200, 242, 288, 289, 338, 361, 392, 400, 484, 529, 576, 578, 676, 722, 784, 800, 841, 961, 968, 1058, 1152, 1156, 1352, 1369, 1444, 1568, 1600, 1681, 1682, 1849, 1922, 1936, 2116, 2209, 2304, 2312, 2704 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers that are the product of a binary power (cf. A000079) and a square of an odd prime, or numbers having exactly 3 odd divisors: A001227(a(n)) = 3. [Reinhard Zumkeller, May 01 2012] Numbers n such that the symmetric representation of sigma(n) has 3 subparts. - Omar E. Pol, Dec 28 2016 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Ron Knott, Runsums T. Verhoeff, Rectangular and Trapezoidal Arrangements, J. Integer Sequences, Vol. 2, 1999, #99.1.6. EXAMPLE a(1)=9 is the smallest number with 3 runsums: 2+3+4=4+5=9. PROG (Haskell) import Data.Set (singleton, deleteFindMin, insert) a072502 n = a072502_list !! (n-1) a072502_list = f (singleton 9) \$ drop 2 a001248_list where    f s (x:xs) = m : f (insert (2 * m) \$ insert x s') xs where                 (m, s') = deleteFindMin s -- Reinhard Zumkeller, May 01 2012 CROSSREFS Not to be confused with A069562. Cf. A001227, A001248, A038547, A038550, A237593, A279387. Sequence in context: A038837 A034046 A069562 * A195268 A227279 A102042 Adjacent sequences:  A072499 A072500 A072501 * A072503 A072504 A072505 KEYWORD easy,nonn AUTHOR Ron Knott, Jan 27 2003 EXTENSIONS Extended by Ray Chandler, Dec 30 2011 STATUS approved

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