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A263299
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Primes that are the concatenation of k 1's, the digits of k^2 + k + 1, and k 1's.
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2
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OFFSET
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1,1
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COMMENTS
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Inspiration was a(6) that is concatenation of 10 1's, 10^2 + 10 + 1 and 10 1's. a(6) is R_23 and A004022(3).
k=1, 3, 4, 5, 6, 10 are initial values that generate primes in sequence. The consecutive central polygonal numbers associated with the four consecutive k are 13, 21, 31 and 43.
Note that the middle term of a(2) is 13, not 3.
Next term is too large to include.
The next term has 513 digits. - Harvey P. Dale, Jan 27 2019
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LINKS
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Chai Wah Wu, Table of n, a(n) for n = 1..8
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EXAMPLE
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131 is in the list because 131 is a concatenation of 1, (1^2 + 1 + 1) = 3 and 1, and because 131 is prime.
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MATHEMATICA
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Select[FromDigits/@Table[Join[PadRight[{}, n, 1], IntegerDigits[n^2+n+1], PadRight[{}, n, 1]], {n, 20}], PrimeQ] (* Harvey P. Dale, Jan 27 2019 *)
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PROG
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(PARI) for(n=1, 1e3, if(isprime(k=eval(Str((10^n - 1)/9, n^2 + n + 1, (10^n - 1)/9))), print1(k", ")))
(Python)
from gmpy2 import is_prime
A263299_list = [n for n in (int('1'*k+str(k*(k+1)+1)+'1'*k) for k in range(10**2)) if is_prime(n)] # Chai Wah Wu, Oct 19 2015
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CROSSREFS
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Cf. A002061, A002275, A004022, A068817, A070220, A070746, A261364, A262399.
Sequence in context: A033530 A222876 A267720 * A330202 A243832 A331909
Adjacent sequences: A263296 A263297 A263298 * A263300 A263301 A263302
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KEYWORD
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nonn,base
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AUTHOR
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Altug Alkan, Oct 13 2015
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STATUS
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approved
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