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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
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
Sequence in context: A032913 A353397 A261072 * A073573 A309328 A059621
KEYWORD
nonn,base,less
AUTHOR
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 March 28 12:26 EDT 2024. Contains 371254 sequences. (Running on oeis4.)