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EXAMPLE
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For n = 4 the a(4) = 18 solutions are given below. The partial order is assumed to be strict; for the non-strict case, the elements (1,1), (2,2), (3,3), (4,4) should be added to each list.
P1 = {(1,3), (2,3), (2,4)},
P2 = {(1,4), (2,4), (3,4)},
P3 = {(1,4), (2,3), (2,4)},
P4 = {(1,4), (2,3), (2,4), (3,4)},
P5 = {(1,2), (1,4), (3,4)},
P6 = {(1,2), (1,4), (2,4), (3,4)},
P7 = {(1,3), (1,4), (2,3)},
P8 = {(1,3), (1,4), (2,4)},
P9 = {(1,3), (1,4), (2,4), (3,4)},
P10 = {(1,3), (1,4), (2,3), (2,4)},
P11 = {(1,3), (1,4), (2,3), (2,4), (3,4)},
P12 = {(1,2), (1,3), (1,4)},
P13 = {(1,2), (1,3), (1,4), (3,4)},
P14 = {(1,2), (1,3), (1,4), (2,3)},
P15 = {(1,2), (1,3), (1,4), (2,4)},
P16 = {(1,2), (1,3), (1,4), (2,4), (3,4)},
P17 = {(1,2), (1,3), (1,4), (2,3), (2,4)},
P18 = {(1,2), (1,3), (1,4), (2,3), (2,4), (3,4)}.
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