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%I #17 Nov 29 2016 17:51:35
%S 11,19,41,61,89,101,109,127,139,149,151,157,163,181,191,193,199,211,
%T 229,241,251,269,271,281,331,349,359,389,401,409,421,433,439,449,457,
%U 461,463,487,491,499,509,521,541,569,571,601,631,641,659,661,677,691,701,709,751,761,769,809
%N Prime numbers that can be written as concatenation of two nonprimes in decimal representation.
%C This is not A066738 as we concatenate exactly two nonprimes here.
%C A121609 is the dual sequence where "prime" and "nonprime" are switched in the definition.
%H Jean-Marc Falcoz, <a href="/A278799/b278799.txt">Table of n, a(n) for n = 1..7135</a>
%e 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.
%o (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
%Y Cf. A066738, A121609.
%K nonn,base
%O 1,1
%A _Eric Angelini_ and _Jean-Marc Falcoz_, Nov 28 2016
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