A068646 - OEIS

%I #20 Jan 17 2019 13:44:05

%S 11,151,15551,155555555555555555551,155555555555555555555555555555551

%N Primes in which a string of 5's is sandwiched between two 1's.

%C The next term is too big to include. - _Sascha Kurz_, Mar 19 2002

%H Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp151">PDP Reference Table - 151</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15551.htm#prime">Prime numbers of the form 155...551</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%e 11 is also a member in which a string of 0 5's is there between two one's.

%p a := 1:b := 5: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);

%t Select[Table[FromDigits[Join[PadRight[{1},n,5],{1}]],{n,500}],PrimeQ] (* _Harvey P. Dale_, Aug 08 2012 *)

%Y Cf. A068645-A068651, A056246, A082699.

%K base,nonn

%O 1,1

%A _Amarnath Murthy_, Feb 28 2002

%E More terms from _Sascha Kurz_, Mar 19 2002

%E Edited by _Ray Chandler_, Nov 04 2014