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1, 7, 18, 2, 14, 5, 4, 28, 10, 8, 25, 20, 16, 19, 9, 1, 7, 18, 2, 14, 5, 4, 28, 10, 8, 25, 20, 16, 19, 9, 1, 7, 18, 2, 14, 5, 4, 28, 10, 8, 25, 20, 16, 19, 9, 1, 7, 18, 2, 14, 5, 4, 28, 10, 8, 25, 20, 16, 19, 9, 1, 7, 18, 2, 14, 5, 4, 28, 10, 8, 25, 20, 16, 19, 9, 1, 7, 18, 2, 14, 5, 4
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refs;
listen;
history;
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OFFSET
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0,2
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COMMENTS
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Sequence is periodic with period length of 15. That a(15) = 1 means that 31 is not prime in Z[sqrt(7)], being factorable as (-1)(9 - 4sqrt(7))(9 + 4sqrt(7)). - Alonso del Arte, Oct 11 2012
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LINKS
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Table of n, a(n) for n=0..81.
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FORMULA
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a(n) = +a(n - 15). G.f.: ( -1 - 7*x - 18*x^2 - 2*x^3 - 14*x^4 - 5*x^5 - 4*x^6 - 28*x^7 - 10*x^8 - 8*x^9 - 25*x^10 - 20*x^11 - 16*x^12 - 19*x^13 - 9*x^14 ) / ( (x - 1)*(1 + x^4 + x^3 + x^2 + x)*(1 + x + x^2)*(1 - x + x^3 - x^4 + x^5 - x^7 + x^8) ). [R. J. Mathar, Apr 20 2010]
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MATHEMATICA
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PowerMod[7, Range[0, 90], 31] (* Harvey P. Dale, Jul 23 2011 *)
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PROG
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(Sage) [power_mod(7, n, 31)for n in xrange(0, 82)] # [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Nov 27 2009]
(MAGMA)[7^n mod 31: n in [0..80]]; // Vincenzo Librandi, Feb 08 2011
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CROSSREFS
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Sequence in context: A138491 A022511 A113613 * A034083 A185455 A103570
Adjacent sequences: A070412 A070413 A070414 * A070416 A070417 A070418
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KEYWORD
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nonn,easy
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AUTHOR
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N. J. A. Sloane, May 12 2002
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STATUS
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approved
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