OFFSET
1,2
COMMENTS
Also numbers which are not of the form n = (r+1)(2a+3r)/2 for any positive integers r and a >= 1.
Except a(3) = 3, these are the powers of 2 and the products of a power of two 2^k with an odd prime p such that 1+2^(k+1)/3 <= p <= 3(2^(k+1)-1). For example, 20 is in the sequence as 20 = 2^2*5 and 1+2^3/3 <= 5 <= 3(2^3-1).
The equivalent sequence for arithmetic progressions with a common difference of 2 is A000040, the prime numbers (i.e., the numbers > 1 which are not sum of positive integers in arithmetic progression with a common difference 2 are exactly the primes).
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000 (first 121 terms from Jean-Christophe Hervé)
J. W. Andrushkiw, R. I. Andrushkiw and C. E. Corzatt, Representations of Positive Integers as Sums of Arithmetic Progressions, Mathematics Magazine, Vol. 49, No. 5 (Nov., 1976), pp. 245-248.
Francisco Javier de Vega, Some Variants of Integer Multiplication, Axioms (2023) Vol. 12, 905. See p. 8.
M. A. Nyblom and C. Evans, On the enumeration of partitions with summands in arithmetic progression, Australian Journal of Combinatorics, Vol. 28 (2003), pp. 149-159.
FORMULA
A243223(a(n))) = 0.
EXAMPLE
5 is not in the sequence because 5 = 1+4.
CROSSREFS
KEYWORD
nonn,look
AUTHOR
Jean-Christophe Hervé, Jun 01 2014
STATUS
approved