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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

Table of n, a(n) for n=1..10.

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 CaƱestro 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.)