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A068649 Primes in which a string of 9's is sandwiched between two 1's. 2
11, 191, 19991, 199999991, 19999999999999999999999999999999999999991, 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The next term has 199 9's sandwiched between the starting and ending 1.

LINKS

Table of n, a(n) for n=1..6.

Patrick De Geest, PDP Reference Table - 191.

Makoto Kamada, Prime numbers of the form 199...991.

Index entries for primes involving repunits.

EXAMPLE

11 is also a member in which a string of 0 9's is there between two one's.

MAPLE

a := 1:b := 9:i := 1:for n from 0 to 500 do c := a+10*(10^n-1)/9*b+10^(n+1)*a; if(isprime(c)) then d[i] := c; i := i+1; end if; end do:q := seq(d[j], j=1..i-1);

MATHEMATICA

Select[Table[FromDigits[Join[{1}, Table[9, {i}], {1}]], {i, 0, 200}], PrimeQ]

CROSSREFS

Cf. A056250, A068645, A068646, A068647, A068648, A068650, A068651, A082703.

Sequence in context: A002195 A280070 A171553 * A158509 A072290 A258295

Adjacent sequences:  A068646 A068647 A068648 * A068650 A068651 A068652

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Feb 28 2002

EXTENSIONS

More terms from Sascha Kurz and Harvey P. Dale, Mar 19 2002

Edited by Ray Chandler, Nov 04 2014

STATUS

approved

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Last modified August 24 22:49 EDT 2019. Contains 326314 sequences. (Running on oeis4.)