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A154508
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Numbers n if concatenation of n-th nonprime and n = prime.
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0
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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; internal format)
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OFFSET
| 1,1
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COMMENTS
| Indices n such that the concatenation A141468(n)//n yields a prime. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 12 2009]
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EXAMPLE
| If 4=nonprime(n=3) and 43=prime, then 3=a(1). If 10=nonprime(n=7) and 107=prime, then 7=a(2). If 14=nonprime(n=9) and 149=prime, then 9=a(3). If 27=nonprime(n=23) and 3323=prime, then 23=a(4). If 56=nonprime(n=41) and 5641=prime, then 41=a(5), etc.
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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: [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 12 2009]
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CROSSREFS
| Cf. A000040, A141468.
Sequence in context: A110674 A003528 A032913 * A073573 A059621 A034926
Adjacent sequences: A154505 A154506 A154507 * A154509 A154510 A154511
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KEYWORD
| nonn,base,uned
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AUTHOR
| Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Jan 11 2009
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EXTENSIONS
| Extended by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 12 2009
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