A120389 a(n) is such that the a(n)-th composite number is (n-th prime)^2.

1, 4, 15, 33, 90, 129, 227, 288, 429, 694, 798, 1149, 1417, 1565, 1879, 2399, 2993, 3201, 3879, 4365, 4623, 5429, 6002, 6920, 8245, 8948, 9314, 10067, 10457, 11245, 14251, 15184, 16627, 17130, 19711, 20253, 21919, 23653, 24845, 26687, 28604

Offset: 1

Author

Leroy Quet, Jun 30 2006

Keywords

• nonn

Examples

			a(1)=1 because the 1st composite is 4 = 2^2 = (1st prime)^2.
a(4)=33 because the 33rd composite is 49 = 7^2 = (4th prime)^2;
		

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002808.

Programs

Maple

c:=proc(n) if isprime(n)=false then n else fi end: C:=[seq(c(n),n=2..53000)]: a:=proc(n) local ct,i: ct:=0: for i from 1 while C[i]<=ithprime(n)^2 do ct:=ct+1: od: end: seq(a(n),n=1..50); # Emeric Deutsch, Jul 26 2006

Python

from sympy import prime, compositepi
A120389_list = [compositepi(prime(i)**2) for i in range(1,101)] # Chai Wah Wu, Apr 21 2018

Formula

a(n) = A065855(A000040(n)^2).

Extensions

More terms from Emeric Deutsch, Jul 26 2006