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A278799
Prime numbers that can be written as concatenation of two nonprimes in decimal representation.
1
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
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
Sequence in context: A033201 A154386 A066738 * A337508 A081027 A322474
KEYWORD
nonn,base
AUTHOR
STATUS
approved