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A072502
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Numbers that are run sums (trapezoidal, the difference between two triangular numbers) in exactly 3 ways.
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13
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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
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OFFSET
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1,1
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COMMENTS
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Also numbers that are the product of a power of 2 (A000079) and the 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
Also numbers that can be expressed as the sum of k > 1 consecutive positive integers in exactly 2 ways. E.g., 2+3+4 = 9 and 4+5 = 9, 3+4+5+6 = 18 and 5+6+7 = 18. - Julie Jones, Aug 13 2018
Appears to be numbers n such that tau(2*n) = tau(n) + 3. - Gary Detlefs, Jan 22 2020
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = 2 * Sum_{p odd prime} 1/p^2 = 2 * A085548 - 1/2 = 0.404494... - Amiram Eldar, Feb 18 2021
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EXAMPLE
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a(1)=9 is the smallest number with 3 run sums: 2+3+4 = 4+5 = 9.
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PROG
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(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
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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