A068649
Primes in which a string of 9's is sandwiched between two 1's.
Original entry on oeis.org
11, 191, 19991, 199999991, 19999999999999999999999999999999999999991, 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991
Offset: 1
11 is also a member in which a string of 0 9's is there between two one's.
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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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Select[Table[FromDigits[Join[{1}, Table[9, {i}], {1}]], {i, 0, 200}], PrimeQ]
A056250
Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 81 for n > 0.
Original entry on oeis.org
0, 1, 3, 7, 39, 85, 199, 729, 1459, 23671, 28629
Offset: 1
191 is prime, hence 1 is a term.
- Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
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Select[Range[0, 2000], PrimeQ[(180 10^# - 81) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)
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a=11;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+81)
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for(n=0,1500,if(isprime((180*10^n-81)/9),print1(n,",")))
Additional comments from
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
A246804
Numbers k such that (10^(k+2) + 999) * 10^k + 1 is prime.
Original entry on oeis.org
1, 3, 15, 135, 645, 1373, 195317, 237249
Offset: 1
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[n: n in [1..500] | IsPrime((10^(n+2)+999)*10^n+1)];
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A246804:=n->`if`(isprime((10^(n+2)+999)*10^n+1), n, NULL): seq(A246804(n), n=1..10^3); # Wesley Ivan Hurt, Nov 16 2014
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Select[Range[10^3], PrimeQ[(10^(# + 2) + 999)*10^# + 1] &]
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for( n=1,9999, ispseudoprime((10^(n+2)+999)*10^n+1) & print1(n","))
Showing 1-3 of 3 results.
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