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A069690
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Primes that yield another prime on placing a 9 on both sides (as leading and trailing digits).
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7
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2, 19, 23, 31, 41, 43, 47, 53, 61, 67, 71, 73, 83, 101, 107, 109, 113, 149, 163, 193, 211, 239, 241, 263, 269, 277, 313, 317, 331, 347, 373, 397, 409, 421, 439, 443, 499, 521, 523, 541, 547, 607, 617, 619, 641, 647, 673, 677, 757, 787, 829, 863, 877, 907, 911
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OFFSET
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1,1
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LINKS
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EXAMPLE
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241 belongs to this sequence as 92419 is also a prime.
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MAPLE
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filter:= proc(n) isprime(9+10*n+9*10^(2+ilog10(n))) end proc:
select(filter, [seq(ithprime(i), i=1..1000)]); # Robert Israel, Feb 02 2021
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MATHEMATICA
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Select[ Range[2000], PrimeQ[ # ] && PrimeQ[ FromDigits[ Insert[ IntegerDigits[ # ], 9, {{1}, {-1}}]]] &]
Select[Prime[Range[200]], PrimeQ[FromDigits[Join[{9}, IntegerDigits[#], {9}]]]&] (* Harvey P. Dale, Nov 07 2022 *)
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PROG
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(Python)
from sympy import isprime, primerange
def ok(p): return isprime(int('9'+str(p)+'9'))
def aupto(lim): return [p for p in primerange(2, lim+1) if ok(p)]
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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