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A142349
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Primes of the form n/4*(c(n)-r(n)), where c(n)=n-th composite and r(n)=n-th nonprime.
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0
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2, 3, 5, 7, 5, 5, 7, 11, 17, 23, 37, 41, 43, 29, 31, 61, 41, 43, 67, 71, 73, 79, 53, 83, 89, 97, 67, 107, 109, 113, 131, 67, 137, 139, 149, 151, 101, 163, 109, 167, 113, 173, 181, 191, 193, 197, 199, 223, 229, 233, 157, 239, 241, 163, 257, 173, 263, 181, 281, 193, 293
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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EXAMPLE
| If n=24, then 24/4*(c(24)-r(24))=6/(36-34)=2=a(1).
If n=40, then 40/4*(c(40)-r(40))=10/(48-44)=5=a(2).
If n=42, then 42/4*(c(42)-r(42))=42/4*(48-45)=7=a(3).
If n=60, then 60/4*(c(60)-r(60))=15/(84-81)=5=a(4).
If n=80, then 80/4*(c(80)-r(80))=20/(110-106)=5=a(5), etc.
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MAPLE
| A141468 := proc(n) option remember ; if n = 1 then 0; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a) ; fi; od: fi; end: A002808 := proc(n) option remember ; A141468(n+2) ; end: for n from 1 to 3000 do p := n/(A002808(n)-A141468(n))/4 ; if type(p, 'integer') then if isprime(p) then printf("%d, ", p) ; fi; fi; od: [From R. J. Mathar (mathar(AT)strw.leideuniv.nl), Jan 23 2009]
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CROSSREFS
| Cf. A002808, A141468.
Sequence in context: A080164 A182949 A126048 * A081622 A064143 A115274
Adjacent sequences: A142346 A142347 A142348 * A142350 A142351 A142352
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KEYWORD
| nonn
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AUTHOR
| Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Sep 21 2008
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EXTENSIONS
| Corrected and extended by R. J. Mathar (mathar(AT)strw.leideuniv.nl), Jan 23 2009
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