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A379900 a(n) = position of prime(n) in A379899, or a(n) = -1 if prime(n) is not in A379899.

%I A379900 #8 Jan 06 2025 04:12:41
%S A379900 1,2,5,3,4,6,7,12,13,8,14,9,10,15,16,11,17,31,18,19,32,20,21,33,34,35,
%T A379900 22,23,36,37,24,25,38,26,39,27,40,28,29,41,30,42,73,43,44,74,75,76,77,
%U A379900 45,46,78,47,79,48,80,49,81,50,51,82,52,83,84,53,54,85
%N A379900 a(n) = position of prime(n) in A379899, or a(n) = -1 if prime(n) is not in A379899.
%H A379900 Robert C. Lyons, <a href="/A379900/b379900.txt">Table of n, a(n) for n = 1..10000</a>
%o A379900 (Python)
%o A379900 from sympy import prime, primefactors, primepi
%o A379900 def get_a379900(a379899_indices):
%o A379900     a379900 = []
%o A379900     count = 1
%o A379900     while True:
%o A379900         p = prime(count)
%o A379900         if p not in a379899_indices:
%o A379900             break
%o A379900         a379900.append(a379899_indices[p])
%o A379900         count += 1
%o A379900     return a379900
%o A379900 a379899 = [2]
%o A379900 a379899_indices = dict()
%o A379900 a379899_indices[2] = 1
%o A379900 max_a379899_len=1000
%o A379900 while len(a379899) <= max_a379899_len:
%o A379900     candidate = a379899[-1]
%o A379900     done = False
%o A379900     while not done:
%o A379900         candidate = candidate + 4
%o A379900         factors = primefactors(candidate)
%o A379900         for factor in factors:
%o A379900             if factor not in a379899_indices:
%o A379900                 a379899.append(factor)
%o A379900                 a379899_indices[factor] = len(a379899)
%o A379900                 done = True
%o A379900                 break
%o A379900 a379900 = get_a379900(a379899_indices)
%o A379900 print(a379900)
%Y A379900 Cf. A379899.
%K A379900 nonn,easy
%O A379900 1,2
%A A379900 _Robert C. Lyons_, Jan 05 2025