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A218721
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a(n) = (18^n-1)/17.
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35
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0, 1, 19, 343, 6175, 111151, 2000719, 36012943, 648232975, 11668193551, 210027483919, 3780494710543, 68048904789775, 1224880286215951, 22047845151887119, 396861212733968143, 7143501829211426575, 128583032925805678351
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OFFSET
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0,3
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COMMENTS
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Partial sums of powers of 18 (A001027), q-integers for q=18: diagonal k=1 in triangle A022182.
Except for 0, 1 and 19, all terms are Brazilian repunits numbers in base 18, and so belong to A125134. From n = 3 to n = 8286, all terms are composite. See link "Generalized repunit primes".
As explained in the extensions of A128164, a(25667) = (18^25667 - 1)/17 would be (is) the smallest prime in base 18. (End)
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LINKS
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FORMULA
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a(n) = floor(18^n/17).
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EXAMPLE
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a(3) = (18^3 - 1)/17 = 343 = 7 * 49; a(6) = (18^6 - 1)/17 = 2000719 = 931 * 2149. - Bernard Schott, May 01 2017
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MATHEMATICA
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Join[{0}, Accumulate[18^Range[0, 20]]] (* Harvey P. Dale, Nov 08 2012 *)
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PROG
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(Magma) [n le 2 select n-1 else 19*Self(n-1)-18*Self(n-2): n in [1..20]]; // Vincenzo Librandi, Nov 07 2012
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CROSSREFS
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Cf. A000225, A001027, A002275, A002450, A002452, A003462, A003463, A003464, A014901, A014935, A016123, A016125, A022182, A023000, A023001, A064108, A091030, A091045, A094028, A125134, A128164, A131865, A135518, A135519, A218722, A218724, A218733, A218743, A218752.
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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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