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A075341 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)). 1
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; text; internal format)
OFFSET

1,2

COMMENTS

a(k) = b(k-5) for k > 8 where b(r) is the r-th term of A075340.

LINKS

Table of n, a(n) for n=1..40.

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 *)

CROSSREFS

Cf. A075340.

See also: Always look for prime: A061092. Always look for composite: A061766.

Sequence in context: A274552 A280682 A140293 * A309358 A143789 A068521

Adjacent sequences:  A075338 A075339 A075340 * A075342 A075343 A075344

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Sep 18 2002

EXTENSIONS

More terms from David Wasserman, Jan 16 2005

STATUS

approved

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Last modified April 8 08:54 EDT 2020. Contains 333313 sequences. (Running on oeis4.)