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A278799
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Prime numbers that can be written as concatenation of two nonprimes in decimal representation.
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1
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11, 19, 41, 61, 89, 101, 109, 127, 139, 149, 151, 157, 163, 181, 191, 193, 199, 211, 229, 241, 251, 269, 271, 281, 331, 349, 359, 389, 401, 409, 421, 433, 439, 449, 457, 461, 463, 487, 491, 499, 509, 521, 541, 569, 571, 601, 631, 641, 659, 661, 677, 691, 701, 709, 751, 761, 769, 809
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OFFSET
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1,1
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COMMENTS
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This is not A066738 as we concatenate exactly two nonprimes here.
A121609 is the dual sequence where "prime" and "nonprime" are switched in the definition.
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LINKS
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EXAMPLE
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11 (prime) is the concatenation of "1" (nonprime) and "1" (nonprime); the next prime term cannot be 13 as "3" is a concatenated prime; the next prime term cannot be 17 as "7" is a concatenated prime; the next prime term is 19 as "1" and "9" are both nonprimes; the next prime term cannot be less than 41 because all terms < 41 and > 19 start with either a "2" or a "3", which are primes; etc.
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PROG
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(PARI) is(n)=if(!isprime(n), return(0)); my(d=digits(n)); for(i=2, #d, if(d[i] && !isprime(fromdigits(d[1..i-1])) && !isprime(fromdigits(d[i..#d])), return(1))); 0 \\ Charles R Greathouse IV, Nov 28 2016
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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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STATUS
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approved
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