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A075341
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a(1) = 1, a(2n) is the smallest composite number == 1 mod (a(2n-1)) and a(2n+1) is the smallest prime == 1 (mod a(2n)).
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1
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1, 4, 5, 6, 7, 8, 17, 18, 19, 20, 41, 42, 43, 44, 89, 90, 181, 182, 547, 548, 1097, 1098, 7687, 7688, 15377, 15378, 30757, 30758, 276823, 276824, 553649, 553650, 2768251, 2768252, 8304757, 8304758, 99657097, 99657098, 199314197, 199314198
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| a(k) = b(k-5) for k > 8 where b(r) is the r-th term of A075340.
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MATHEMATICA
| a[1] = 1; a[2] = 4; a[n_] := a[n] = Block[{k = a[n - 1] + 1, m = a[n - 1]}, If[OddQ@n, While[ !PrimeQ@k || Mod[k, m] != 1, k += m]; k, While[PrimeQ@k || Mod[k, m] != 1, k += m]; k]]; Array[a, 40] (* Robert G. Wilson v Sep 21 2006 *)
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CROSSREFS
| Cf. A075340.
See also: Always look for prime: A061092. Always look for composite: A061766.
Sequence in context: A059709 A071623 A140293 * A143789 A068521 A196697
Adjacent sequences: A075338 A075339 A075340 * A075342 A075343 A075344
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KEYWORD
| nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Sep 18 2002
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EXTENSIONS
| More terms from David Wasserman (dwasserm(AT)earthlink.net), Jan 16 2005
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