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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: A003528 A032913 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 August 5 22:41 EDT 2021. Contains 346488 sequences. (Running on oeis4.)