A183187
Numbers k such that 10^(2k+1)-10^k-1 is prime.
Original entry on oeis.org
26, 378, 1246, 1798, 2917, 23034, 47509, 52140, 67404
Offset: 1
- C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
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Do[If[PrimeQ[10^(2n + 1) - 10^n - 1], Print[n]], {n, 3000}]
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for(n=1,1e3,if(ispseudoprime(10^(2*n+1)-10^n-1),print1(n", "))) \\ Charles R Greathouse IV, Jul 15 2011
A181965
a(n) = 10^(2n+1) - 10^n - 1.
Original entry on oeis.org
8, 989, 99899, 9998999, 999989999, 99999899999, 9999998999999, 999999989999999, 99999999899999999, 9999999998999999999, 999999999989999999999, 99999999999899999999999, 9999999999998999999999999, 999999999999989999999999999, 99999999999999899999999999999, 9999999999999998999999999999999
Offset: 0
- Patrick De Geest, Palindromic Wing Primes: (9)8(9), updated: June 25, 2017.
- Makoto Kamada, Factorization of 99...99899...99, updated Dec 11 2018.
- Markus Tervooren, Factorizations of (9)w8(9)w, FactorDB.com
- Index entries for linear recurrences with constant coefficients, signature (111,-1110,1000).
Cf.
A138148 (cyclops numbers with binary digits only),
A002113 (palindromes).
Cf.
A332190 ..
A332197 (variants with different middle digit 0, ..., 7).
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A181965 := n -> 10^(2*n+1)-1-10^n; # M. F. Hasler, Feb 08 2020
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Array[10^(2 # + 1) - 1- 10^# &, 15, 0] (* M. F. Hasler, Feb 08 2020 *)
Table[With[{c=PadRight[{},n,9]},FromDigits[Join[c,{8},c]]],{n,0,20}] (* Harvey P. Dale, Jun 07 2021 *)
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apply( {A181965(n)=10^(n*2+1)-1-10^n}, [0..15]) \\ M. F. Hasler, Feb 08 2020
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def A181965(n): return 10**(n*2+1)-1-10^n # M. F. Hasler, Feb 08 2020
Edited and extended to a(0) = 8 by
M. F. Hasler, Feb 10 2020
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