A068649 Primes in which a string of 9's is sandwiched between two 1's.
11, 191, 19991, 199999991, 19999999999999999999999999999999999999991, 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991
Offset: 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.
Programs
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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);
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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
Comments