A280029 a(1)=3; thereafter, if n odd, a(n) = a(n-1)-st prime, and if n even, a(n) = a(n-1)-st nonprime.

3, 6, 13, 21, 73, 98, 521, 636, 4721, 5439, 53323, 59318, 737183, 801235, 12215237, 13068626, 238205651, 251987681, 5381022817, 5644769560, 138933578143, 144807081479, 4053430178401, 4203399920996, 132332669898119, 136669373292795, 4793783797371011

Offset: 1

Author

N. J. A. Sloane, Dec 25 2016

Keywords

• nonn

Comments

This is the sequence S_2 mentioned in A141436. The primes and nonprimes alternate.

Crossrefs

Cf. A000040, A018252, A141436, A280028, A280030.

Programs

Maple

# See A280028 for Maple program

Mathematica

nonPrime[n_Integer] := FixedPoint[n + PrimePi@# &, n + PrimePi@ n]; a[n_] := a[n] = If[OddQ@ n, Prime@ a[n -1], nonPrime[ a[n -1]]]; a[1] = 3; Array[a, 22] (* Robert G. Wilson v, Dec 28 2016 *)

Extensions

a(12)-a(26) from Hans Havermann, Dec 25 2016

a(27) from Chai Wah Wu, Dec 27 2016