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

%I A120389 #16 Apr 21 2018 17:36:53
%S A120389 1,4,15,33,90,129,227,288,429,694,798,1149,1417,1565,1879,2399,2993,
%T A120389 3201,3879,4365,4623,5429,6002,6920,8245,8948,9314,10067,10457,11245,
%U A120389 14251,15184,16627,17130,19711,20253,21919,23653,24845,26687,28604
%N A120389 a(n) is such that the a(n)-th composite number is (n-th prime)^2.
%H A120389 Chai Wah Wu, <a href="/A120389/b120389.txt">Table of n, a(n) for n = 1..10000</a>
%F A120389 a(n) = A065855(A000040(n)^2).
%e A120389 a(1)=1 because the 1st composite is 4 = 2^2 = (1st prime)^2.
%e A120389 a(4)=33 because the 33rd composite is 49 = 7^2 = (4th prime)^2;
%p A120389 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
%o A120389 (Python)
%o A120389 from sympy import prime, compositepi
%o A120389 A120389_list = [compositepi(prime(i)**2) for i in range(1,101)] # _Chai Wah Wu_, Apr 21 2018
%Y A120389 Cf. A002808.
%K A120389 nonn
%O A120389 1,2
%A A120389 _Leroy Quet_, Jun 30 2006
%E A120389 More terms from _Emeric Deutsch_, Jul 26 2006