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 A054416 Numbers n such that 9090...9091 (with n-1 copies of 90 and one copy of 91) is prime. 3
 2, 3, 9, 15, 26, 33, 146, 320, 1068, 1505 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES J. A. H. Hunter and J. S. Madachy, Mathematical Diversions, New York: Dover Publications, Inc., 1974, pp. 4-5. Originally published by Van Nostrand, 1963. LINKS D. Broadhurst, Proof that 1505 term is prime FORMULA 10*(10^(2n)-1)/11 + 1 is prime. EXAMPLE The first 3 numbers are 9091, 909091, 909090909090909091 MATHEMATICA Do[ If[ PrimeQ[ 10*(10^(2n) - 1)/11 + 1], Print[ n ] ], {n, 0, 1505} ] Position[Table[FromDigits[PadLeft[{9, 1}, 2n, {9, 0}]], {n, 1510}], _?PrimeQ]// Flatten (* Harvey P. Dale, Nov 02 2017 *) CROSSREFS Equals (A001562-1)/2. Sequence in context: A061933 A251239 A124881 * A291868 A092638 A206777 Adjacent sequences:  A054413 A054414 A054415 * A054417 A054418 A054419 KEYWORD nonn AUTHOR Antreas P. Hatzipolakis (xpolakis(AT)otenet.gr), May 22 2000 EXTENSIONS More terms from Michael Kleber and Harvey Dubner (harvey(AT)dubner.com), May 22, 2000 Ignacio Larrosa CaĆ±estro reports that the 1068 term has now been established to be a prime using Titanix 1.01, Oct 23 2000 STATUS approved

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Last modified July 23 00:43 EDT 2019. Contains 325228 sequences. (Running on oeis4.)