A082702 -id:A082702 - cp's OEIS frontend

A068648 Primes in which a string of 8's is sandwiched between two 1's.

11, 181, 188888881, 188888888888881, 18888888888888888888888888888888888888881, 188888888888888888888888888888888888888888888888888888888888888888888888888888888888888888881

Offset: 1

Amarnath Murthy, Feb 28 2002

The next term -- a(7) -- has 129 digits. - Harvey P. Dale, Jul 08 2020

			11 is also a member in which a string of 0 8's is there between two one's.

Patrick De Geest, PDP Reference Table - 181.
Makoto Kamada, Prime numbers of the form 188...881.
Index entries for primes involving repunits.

Cf. A068645-A068651, A056249, A082702.

a := 1:b := 8: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);

Select[Table[10FromDigits[PadRight[{1},n,8]]+1,{n,100}],PrimeQ] (* Harvey P. Dale, Jul 08 2020 *)

More terms from Sascha Kurz, Mar 19 2002

Edited by Ray Chandler, Nov 04 2014

A056249 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0.

Original entry on oeis.org

0, 1, 7, 13, 39, 91, 127, 883, 9423, 14767, 19257, 31233

Offset: 1

Robert G. Wilson v, Aug 18 2000

Numbers n such that (170*10^n - 71)/9 is prime.
Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
Numbers corresponding to terms <= 883 are certified primes.

			181 is prime, hence 1 is a term.

Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

Patrick De Geest, PDP Reference Table - 181.
Makoto Kamada, Prime numbers of the form 188...881.
Index entries for primes involving repunits.

Cf. A000533, A002275, A068648, A082702.

Select[Range[0, 2000], PrimeQ[(170 10^# - 71) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)

a=11;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+71)

for(n=0,1500,if(isprime((170*10^n-71)/9),print1(n,",")))

a(n) = A082702(n-1) - 2 for n > 1.

Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
Edited by N. J. A. Sloane, Jun 15 2007
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
Updated and added a link, by Patrick De Geest, Nov 02 2014
Edited by Ray Chandler, Nov 04 2014

cp's OEIS Frontend

A068648 Primes in which a string of 8's is sandwiched between two 1's.

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