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%I #7 Mar 23 2022 12:51:03
%S 0,0,4,8,14,22,30,42,60,74,92,108,130,148,178,206,230,262,288,324,364,
%T 400,442,480,522,562,614,662,712,764,812,868,922,988,1050,1106,1176,
%U 1234,1312,1376,1452,1528,1598,1678,1750,1838,1920,2006,2092,2172,2266
%N Open disk numbers (version 4): a(n) is the number of points (i+j/2,j*sqrt(3)/2), i,j integers (triangular grid) contained in an open disk of diameter n, centered at (1/2,0).
%C a(n)/(n/2)^2 -> Pi*2/sqrt(3).
%Y Cf. A053417, A053456, A053457, A053458.
%K easy,nonn
%O 0,3
%A Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Jan 13 2000
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