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A238209
The total number of 2's in all partitions of n into an odd number of distinct parts.
2
0, 0, 1, 0, 0, 0, 1, 1, 1, 2, 2, 3, 3, 4, 4, 6, 6, 8, 9, 11, 13, 16, 18, 22, 26, 30, 35, 41, 48, 55, 64, 73, 85, 97, 111, 127, 146, 165, 189, 214, 244, 276, 313, 353, 400, 451, 508, 572, 644, 722, 811, 909, 1018, 1139, 1273, 1421, 1586, 1768, 1968, 2191, 2436
OFFSET
0,10
COMMENTS
The g.f. for "number of k's" is (1/2)*(x^k/(1+x^k))*(Product_{n>=1} 1 + x^n) + (1/2)*(x^k/(1-x^k))*(Product_{n>=1} 1 - x^n).
LINKS
FORMULA
a(n) = Sum_{j=1..round(n/4)} A067661(n-(2*j-1)*2) - Sum_{j=1..floor(n/4)} A067659(n-4*j).
G.f.: (1/2)*(x^2/(1+x^2))*(Product_{n>=1} 1 + x^n) + (1/2)*(x^2/(1-x^2))*(Product_{n>=1} 1 - x^n).
EXAMPLE
a(11) = 3 because the partitions in question are: 8+2+1, 6+3+2, 5+4+2.
PROG
(PARI) seq(n)={my(A=O(x^(n-1))); Vec(x^2*(eta(x^2 + A)/(eta(x + A)*(1+x^2)) + eta(x + A)/(1-x^2))/2, -(n+1))} \\ Andrew Howroyd, May 01 2020
CROSSREFS
Column k=2 of A238450.
Sequence in context: A025149 A026799 A185326 * A342096 A210716 A027190
KEYWORD
nonn
AUTHOR
Mircea Merca, Feb 20 2014
EXTENSIONS
Terms a(51) and beyond from Andrew Howroyd, May 01 2020
STATUS
approved