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A109549 Primes of the form aaaa...aa1 where a is 6, 7, 8 or 9. 1
61, 71, 661, 881, 991, 6661, 99991, 9999991, 6666666661, 7777777777771, 666666666666666661, 8888888888888888881, 77777777777777777771, 666666666666666666661, 6666666666666666666661, 77777777777777777777771 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Easy-to-remember large primes can be formed in this manner.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..43

FORMULA

d=6, 7, 8, 9 a(n) = if prime then Sum[d*10^i, {i, 1, m}] + 1

MATHEMATICA

d[n_] = If[5 + Mod[n, 6] > 0, 5 + Mod[n, 6], 1] a = Flatten[Table[Sum[d[k]*10^i, {i, 1, m}] + 1, {m, 1, 50}, {k, 1, 4}]] b = Flatten[Table[If[PrimeQ[a[[i]]] == True, a[[i]], {}], {i, 1, Length[a]}]]

Select[FromDigits/@Flatten[Table[PadLeft[{1}, i, #]&/@{6, 7, 8, 9}, {i, 2, 100}], 1], PrimeQ[#]&] (* Vincenzo Librandi, Dec 12 2011 *)

CROSSREFS

Cf. A051200, A004022, A093176, A093177, A089346, A093174, A092571, A089345, A089347.

Sequence in context: A064229 A180555 A039478 * A139944 A179012 A227980

Adjacent sequences:  A109546 A109547 A109548 * A109550 A109551 A109552

KEYWORD

nonn

AUTHOR

Roger L. Bagula, Jun 26 2005

STATUS

approved

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Last modified November 12 19:19 EST 2018. Contains 317116 sequences. (Running on oeis4.)