A288469 a(n) = n if n is a nonprime, otherwise take the prime index of n and repeat until you get a nonprime which is then a(n).

1, 1, 1, 4, 1, 6, 4, 8, 9, 10, 1, 12, 6, 14, 15, 16, 4, 18, 8, 20, 21, 22, 9, 24, 25, 26, 27, 28, 10, 30, 1, 32, 33, 34, 35, 36, 12, 38, 39, 40, 6, 42, 14, 44, 45, 46, 15, 48, 49, 50, 51, 52, 16, 54, 55, 56, 57, 58, 4, 60, 18, 62, 63, 64, 65, 66, 8, 68, 69, 70, 20, 72, 21, 74, 75, 76, 77, 78, 22, 80, 81, 82, 9, 84, 85, 86

Offset

1,4

Comments

a(n) = 1 for n in A007097. - Robert Israel, Jun 09 2017

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Formula

From Robert Israel, Jun 09 2017: (Start)

a(n) = n + A010051(n)*(a(A000720(n))-n).

a(A114537(n,k)) = A114537(n,1). (End)

Example

For n = 17:  17 is a prime, so you take the prime index of 17 which is 7. 7 is a prime, so you take the prime index of 7 which is 4. 4 is a nonprime, so a(17) = 4.

Maple

f:= proc(n) option remember; if isprime(n) then procname(numtheory:-pi(n)) else n fi end proc:
map(f, [$1..100]); # Robert Israel, Jun 09 2017

Mathematica

Table[If[!PrimeQ@ n, n, NestWhile[PrimePi, n, PrimeQ]], {n, 86}] (* Michael De Vlieger, Jun 09 2017 *)

Prog

(PARI) a(n)=while(isprime(n), n=primepi(n)); n \\ Charles R Greathouse IV, Jun 09 2017

Crossrefs

Cf. A000720, A007097, A010051, A114537.

Sequence in context: A318876 A330356 A329428 * A076717 A200360 A228451

Adjacent sequences: A288466 A288467 A288468 * A288470 A288471 A288472

Keyword

nonn

Author

Peter Weiss, Jun 09 2017

Status

approved