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

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, 22, 23, 36, 37, 24, 25, 38, 26, 39, 27, 40, 28, 29, 41, 30, 42, 73, 43, 44, 74, 75, 76, 77, 45, 46, 78, 47, 79, 48, 80, 49, 81, 50, 51, 82, 52, 83, 84, 53, 54, 85

Offset: 1

Author

Robert C. Lyons, Jan 05 2025

Keywords

• nonn
• easy

Links

• Robert C. Lyons, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A379899.

Programs

Python

from sympy import prime, primefactors, primepi
def get_a379900(a379899_indices):
    a379900 = []
    count = 1
    while True:
        p = prime(count)
        if p not in a379899_indices:
            break
        a379900.append(a379899_indices[p])
        count += 1
    return a379900
a379899 = [2]
a379899_indices = dict()
a379899_indices[2] = 1
max_a379899_len=1000
while len(a379899) <= max_a379899_len:
    candidate = a379899[-1]
    done = False
    while not done:
        candidate = candidate + 4
        factors = primefactors(candidate)
        for factor in factors:
            if factor not in a379899_indices:
                a379899.append(factor)
                a379899_indices[factor] = len(a379899)
                done = True
                break
a379900 = get_a379900(a379899_indices)
print(a379900)