The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 6 13:45 EST 2021. Contains 349563 sequences. (Running on oeis4.)