A278799 Prime numbers that can be written as concatenation of two nonprimes in decimal representation.
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
Offset: 1
Examples
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.
Links
- Jean-Marc Falcoz, Table of n, a(n) for n = 1..7135
Programs
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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
Comments