A280030 - OEIS

A280030

a(1)=5; thereafter, if n odd, a(n) = a(n-1)-st prime, and if n even, a(n) = a(n-1)-st nonprime.

%I #24 Jul 03 2017 01:54:35

%S 5,9,23,34,139,180,1069,1274,10399,11815,125933,138847,1854341,

%T 2003514,32513171,34639222,667169599,703599332,15783876803,

%U 16518738034,425079924023,442239042120,12891534052099,13348915604303,436194466471661

%N a(1)=5; thereafter, if n odd, a(n) = a(n-1)-st prime, and if n even, a(n) = a(n-1)-st nonprime.

%C This is the sequence S_3 mentioned in A141436. The primes and nonprimes alternate.

%p # See A280028 for Maple program

%t nonPrime[n_Integer] := FixedPoint[n + PrimePi@# &, n + PrimePi@n]; a[n_] := If[OddQ@ n, Prime@ a[n -1], nonPrime[ a[n -1]]]; a[1] = 5; Array[a, 24] (* _Robert G. Wilson v_, Dec 28 2016 *)

%Y Cf. A000040, A018252, A141436, A280028, A280029.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Dec 25 2016

%E a(12)-a(23) from _Chai Wah Wu_, Dec 25 2016

%E a(24) from _Hans Havermann_, Dec 25 2016

%E a(25) from _Chai Wah Wu_, Dec 26 2016