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Search: id:A142352
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| A142352 |
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Primes of the form n/2*(c(n)-r(n)), where c(n)=n-th composite and r(n)=n-th nonprime. |
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1
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| 2, 5, 7, 11, 17, 23, 19, 31, 23, 43, 59, 31, 67, 47, 71, 73, 79, 83, 59, 89, 53, 107, 109, 113, 127, 131, 89, 137, 139, 97, 149, 103, 157, 163, 167, 173, 179, 181, 127, 191, 131, 197, 199, 223, 151, 227, 22, 9, 233, 239, 241, 163, 263, 271, 181, 277, 281, 283, 293
(list; graph; listen)
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OFFSET
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1,1
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EXAMPLE
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If n=12, then 12/2*(c(12)-r(12))=6/(21-18)=2=a(1).
If n=30, then 30/2*(c(30)-r(30))=15/(45-42)=5=a(2).
If n=42, then 42/2*(c(42)-r(42))=21/(60-57)=7=a(3).
If n=66, then 66/2*(c(66)-r(66))=33/(91-88)=11=a(4).
If n=68, then 68/2*(c(68)-r(68))=34/(93-91)=17=a(5), etc.
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MAPLE
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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))/2 ; if type(p, 'integer') then if isprime(p) then printf("%d, ", p) ; fi; fi; od: [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 23 2009]
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CROSSREFS
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Cf. A002808, A141468.
Sequence in context: A023213 A162575 A066408 this_sequence A062044 A077128 A106008
Adjacent sequences: A142349 A142350 A142351 this_sequence A142353 A142354 A142355
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KEYWORD
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nonn
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AUTHOR
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Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Sep 21 2008
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EXTENSIONS
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Corrected and extended by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 23 2009
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