OFFSET
1,1
COMMENTS
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
Column 3 of A266531. - Omar E. Pol, Dec 01 2020
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
T. Verhoeff, Rectangular and Trapezoidal Arrangements, J. Integer Sequences, Vol. 2 (1999), Article #99.1.6.
FORMULA
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
EXAMPLE
a(1)=9 is the smallest number with 3 run sums: 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
KEYWORD
easy,nonn
AUTHOR
Ron Knott, Jan 27 2003
EXTENSIONS
Extended by Ray Chandler, Dec 30 2011
STATUS
approved