A068649 Primes in which a string of 9's is sandwiched between two 1's.

11, 191, 19991, 199999991, 19999999999999999999999999999999999999991, 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991

Offset: 1

Author

Amarnath Murthy, Feb 28 2002

Keywords

• base
• nonn

Comments

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

Examples

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

Links

• Patrick De Geest, PDP Reference Table - 191.
• Makoto Kamada, Prime numbers of the form 199...991.
• Index entries for primes involving repunits.

Crossrefs

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

Programs

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]

Extensions

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

Edited by Ray Chandler, Nov 04 2014