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A158138
Number of nondecreasing integer sequences of length 4 with sum zero and sum of absolute values 2n.
1
1, 4, 6, 11, 13, 22, 24, 35, 39, 52, 56, 73, 77, 96, 102, 123, 129, 154, 160, 187, 195, 224, 232, 265, 273, 308, 318, 355, 365, 406, 416, 459, 471, 516, 528, 577, 589, 640, 654, 707, 721, 778, 792, 851, 867, 928, 944, 1009, 1025, 1092, 1110, 1179, 1197, 1270, 1288
OFFSET
1,2
COMMENTS
a(n) = A000041(n)^2 for n<=2
a(n) = A000041(n)^2 - cumulative A000712(2*n-1-length), 0 <= 2*n-1-length <= floor(n/2) [empirical].
LINKS
FORMULA
a(n) = (floor(n/2) + 1)^2 + 2*A069905(n). - Georg Fischer, Apr 20 2022
EXAMPLE
For n = 6, we count the possible concatenations of the 4 pairs in the list (-6,0),(-5,-1),(-4,-2),(-3,-3) with their negative reversed correspondants (starting with (-6,0,0,6)), giving (6/2 + 1)^2 = 16 quadruples, plus the 3 quadruples (-6,1,1,4), (-6,1,2,3), (-6,2,2,2) and their 3 negative reversed correspondants, giving a total of 22 possibilities. - Georg Fischer, Apr 20 2022
PROG
(AWK) # empirical
function a(n) { s=1; for(i=1; i<n; i++) { if(i%2==0)s+=2*int((i+5)/6); else s+=(i+2)+2*int((i+1)/6); } return s; }
CROSSREFS
Cf. A069905, A158139-A158184 (for length 5..50).
Sequence in context: A287565 A163417 A247336 * A096833 A153357 A310591
KEYWORD
nonn
AUTHOR
R. H. Hardin, Mar 13 2009
STATUS
approved