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A002081
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Numbers congruent to {2, 4, 8, 16} (mod 20).
(Formerly M1113 N0426)
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4
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2, 4, 8, 16, 22, 24, 28, 36, 42, 44, 48, 56, 62, 64, 68, 76, 82, 84, 88, 96, 102, 104, 108, 116, 122, 124, 128, 136, 142, 144, 148, 156, 162, 164, 168, 176, 182, 184, 188, 196, 202, 204, 208, 216, 222, 224, 228, 236, 242, 244, 248, 256, 262, 264, 268, 276, 282
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listen;
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OFFSET
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0,1
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COMMENTS
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First differences are periodic, cf. A000689.
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REFERENCES
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N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = 5*n + (1/2)*(3 + (-1)^n)*(-1)^(n(n+1)/2). - Bruno Berselli, Sep 15 2011
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MAPLE
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A002081:=2*(1+2*z**2+2*z**3)/(z**2+1)/(z-1)**2; # conjectured by Simon Plouffe in his 1992 dissertation
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MATHEMATICA
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Flatten[Table[20n + {2, 4, 8, 16}, {n, 0, 14}]] (* Alonso del Arte, Nov 30 2011 *)
LinearRecurrence[{2, -2, 2, -1}, {2, 4, 8, 16}, 57] (* Ray Chandler, Aug 25 2015 *)
Select[Range[300], MemberQ[{2, 4, 8, 16}, Mod[#, 20]]&] (* Harvey P. Dale, Jul 20 2021 *)
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PROG
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(PARI) a(n) = 5*n + [2, -1, -2, 1][(n%4)+1] \\ Ralf Stephan, Jun 08 2005
(PARI) is(n) = n > 0 && setsearch([2, 4, 8, 16], n%20) > 0 \\ Rick L. Shepherd, Aug 17 2016
(Haskell)
a002081 n = a002081_list
a002081_list = filter ((`elem` [2, 4, 8, 16]) . (`mod` 20)) [1..]
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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More terms from Larry Reeves (larryr(AT)acm.org), Jul 31 2000
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STATUS
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approved
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