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A184324
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The number of disconnected k-regular simple graphs on 2k+4 vertices.
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3
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1, 1, 2, 2, 3, 3, 5, 5, 7, 9, 11, 13, 18, 21, 26, 33, 40, 49, 61, 73, 89, 110, 131, 158, 192, 230, 274, 331, 392, 468, 557, 660, 780, 927, 1088, 1284, 1511, 1775, 2076, 2438, 2843, 3323, 3873, 4510, 5238, 6095, 7057, 8182, 9466, 10945, 12626, 14578, 16780, 19323, 22211
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OFFSET
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0,3
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LINKS
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FORMULA
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a(0)=1. For k>0, a(k) = (k+1) mod 2 + A008483(k+3).
Proof: Let C=A068934, D=A068933, and E=A051031. Now a(n) = D(2k+4, k) = C(k+1, k) C(k+3, k) + A000217(C(k+2,k)), from the disconnected Euler transform. C(k+1, k)=1 because K_{k+1} is connected and the unique k-regular graph on k+1 vertices. For k > 1, since D(k+3,k)=0, then C(k+3,k) = E(k+3,k) = E(k+3,2) = A008483(k + 3). Also, for k >0, since D(k+2,k)=0, then C(k+2,k) = E(k+2,k) = E(k+2,1) = (k+1) mod 2. With the examples below and A165652(n)=0 for n < 6 = offset, QED.
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EXAMPLE
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The a(0)=1 graph is 4K_1. The a(1)=1 graph is 3K_2. The a(2)=2 graphs are C_3+C_5 and C_4+C_4.
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PROG
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(Magma) A184324 := func< n | n eq 0 select 1 else (n+1)mod 2 + A008483(n+3) >; // see A008483 for its MAGMA code.
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CROSSREFS
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This sequence is the third highest diagonal of D=A068933: that is a(n)=D(2k+4, k).
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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