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A243374
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Least number k such that n//k and k//n are both prime where // is the concatenation function, or 0 if no such k exists.
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1
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1, 0, 1, 0, 0, 0, 1, 0, 7, 0, 3, 0, 1, 0, 0, 0, 3, 0, 7, 0, 13, 0, 11, 0, 0, 0, 1, 0, 27, 0, 1, 0, 7, 0, 0, 0, 3, 0, 7, 0, 9, 0, 39, 0, 0, 0, 9, 0, 1, 0, 19, 0, 51, 0, 0, 0, 1, 0, 3, 0, 7, 0, 1, 0, 0, 0, 3, 0, 49, 0, 9, 0, 3, 0, 0, 0, 17, 0, 19, 0, 1, 0, 9, 0, 0, 0, 7, 0, 23
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OFFSET
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1,9
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COMMENTS
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If n ends in a 2, 4, 5, 6, 8, or 0, then a(n) = 0. It is conjectured that the converse is true.
a(n) is odd or 0 for all n.
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LINKS
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EXAMPLE
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91 and 19 are not both prime, 92 and 29 are not both prime, 93 and 39 are not both prime, 94 and 49 are not both prime, 95 and 59 are not both prime, 96 and 69 are not both prime, but 97 and 79 are both prime. Thus a(9) = 7.
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PROG
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(PARI) a(n)=for(k=1, 10^4, if(ispseudoprime(eval(concat(Str(n), Str(k)))) && ispseudoprime(eval(concat(Str(k), Str(n)))), return(k)))
n=1; while(n<100, print1(a(n), ", "); n++)
(Python)
import sympy
from sympy import isprime
def a(n):
..for k in range(1, 10**5):
....if isprime(int(str(k)+str(n))) and isprime(int(str(n)+str(k))):
......return k
n = 1
while n < 100:
..print(a(n), end=', ')
..n += 1
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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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