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

1,1

Comments

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.

Links

Jean-Marc Falcoz, Table of n, a(n) for n = 1..7135

Example

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.

Prog

(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

Crossrefs

Cf. A066738, A121609.

Sequence in context: A033201 A154386 A066738 * A337508 A081027 A322474

Adjacent sequences: A278796 A278797 A278798 * A278800 A278801 A278802

Keyword

nonn,base

Author

Eric Angelini and Jean-Marc Falcoz, Nov 28 2016

Status

approved