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A006185
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Number of pair-coverings with largest block size 3.
(Formerly M3266)
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3
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1, 4, 6, 7, 7, 12, 12, 19, 21, 26, 26, 35, 35, 46, 48, 57, 57, 70, 70, 85, 87, 100, 100, 117, 117, 136, 138, 155, 155, 176, 176, 199, 201, 222, 222, 247, 247, 274, 276, 301, 301, 330, 330, 361, 363, 392, 392, 425, 425, 460, 462, 495, 495, 532
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OFFSET
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3,2
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. G. Stanton, J. L. Allston and D. D. Cowan, Pair-coverings with restricted largest block length, Ars Combin., 11 (1981), 85-98.
M. J. Grannell, T. S. Griggs, K. A. S. Quinn, R. G. Stanton, A census of minimum pair-coverings with restricted largest block length, Ars Combin., 52 (1999).
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LINKS
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FORMULA
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a(n) = n*(n-1)/6 if n == 1 or 3 (mod 6), a(n) = n*(n+1)/6 if n == 0 or 2 (mod 6), a(n) = (n^2+n+4)/6 if n == 4 (mod 6), and a(n) = (n^2-n+16)/6 if n == 5 (mod 6). [From Grannell et al.] - Sean A. Irvine, Jan 17 2017
G.f.: -x^3*(1 + 3*x + x^2 - 2*x^3 + 4*x^5 - x^7 - 2*x^4 - x^6 + x^8) / ( (x^2-x+1)*(1+x+x^2)*(1+x)^2*(x-1)^3 ). - R. J. Mathar, Jan 26 2017
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MATHEMATICA
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LinearRecurrence[{1, 1, -1, 0, 0, 1, -1, -1, 1}, {1, 4, 6, 7, 7, 12, 12, 19, 21}, 100] (* Vincenzo Librandi, Jan 27 2017 *)
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PROG
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(Magma) I:=[1, 4, 6, 7, 7, 12, 12, 19, 21]; [n le 9 select I[n] else Self(n-1)+Self(n-2)-Self(n-3)+Self(n-6)-Self(n-7)-Self(n-8)+Self(n-9): n in [1..60]]; // Vincenzo Librandi, Jan 27 2017
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CROSSREFS
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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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