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 A154508 Numbers k such that appending k to the k-th nonprime yields a prime. 0
 3, 7, 9, 19, 23, 41, 43, 53, 57, 59, 63, 67, 97, 141, 147, 177, 221, 237, 239, 251, 281, 289, 291, 299, 313, 333, 339, 343, 363, 367, 373, 379, 383, 449, 457, 459, 463, 467, 489, 497, 503, 507, 527, 529, 539, 547, 563, 569, 579, 583, 597, 599, 601, 603, 607 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Previous title: "Numbers n if concatenation of n-th nonprime and n = prime." Indices n such that the concatenation A141468(n)//n yields a prime. - R. J. Mathar, Jan 12 2009 LINKS Table of n, a(n) for n=1..55. EXAMPLE Since 4=nonprime(3) and 43 is prime, 3 is in this sequence. Since 10=nonprime(7) and 107 is prime, 7 is in this sequence. Since 14=nonprime(9) and 149 is prime, 9 is in this sequence. Since 27=nonprime(23) and 3323 is prime, 23 is in this sequence. Since 56=nonprime(41) and 5641 is prime, 41 is in this sequence, etc. The sequence A141468 concatenated with the positive nonzero integers begins 1, 12, 43, 64, 85, 96, 107, 128, 149, 1510, 1611, ... Among these terms, 43, 107, and 149 are primes, so 3, 7, and 9 are in this sequence. - Charlie Neder, Dec 23 2018 MAPLE A141468 := proc(n) option remember ; local a; if n <=2 then n-1 ; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a) ; fi; od: fi; end: cat2 := proc(a, b) local d; d := max(1, ilog10(b)+1) ; a*10^d+b ; end: for n from 1 to 1000 do p := cat2( A141468(n), n) ; if isprime(p) then printf("%d, ", n ) ; fi; od: # R. J. Mathar, Jan 12 2009 CROSSREFS Cf. A000040, A141468. Sequence in context: A032913 A353397 A261072 * A073573 A309328 A059621 Adjacent sequences: A154505 A154506 A154507 * A154509 A154510 A154511 KEYWORD nonn,base,less AUTHOR Juri-Stepan Gerasimov, Jan 11 2009 EXTENSIONS Extended by R. J. Mathar, Jan 12 2009 Title corrected by Charlie Neder, Dec 23 2018 STATUS approved

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Last modified September 10 16:40 EDT 2024. Contains 375791 sequences. (Running on oeis4.)