A354148 Index of prime(n) in A090252.

2, 3, 4, 6, 8, 9, 10, 12, 13, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 30, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 61, 62, 64, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 97, 98, 99, 100

Offset

1,1

Comments

All the primes appear in A090252 and in their correct order.

Links

N. J. A. Sloane, Table of n, a(n) for n = 1..9800

Example

A090252 begins 1, 2, 3, 5, 4, 7, 9, ..., so the indices of the primes are 2, 3, 4, 6, ...

Prog

(Python)
from math import gcd, prod
from sympy import isprime
from itertools import count, islice
def agen(): # generator of terms
    alst, aset, mink = [1], {1}, 2
    for n in count(2):
        k, s = mink, n - n//2
        prodall = prod(alst[n-n//2-1:n-1])
        while k in aset or gcd(prodall, k) != 1: k += 1
        alst.append(k); aset.add(k)
        if isprime(k): yield n
        while mink in aset: mink += 1
print(list(islice(agen(), 83))) # Michael S. Branicky, May 23 2022

Crossrefs

Cf. A090252, A354146.

Sequence in context: A156657 A026431 A026433 * A015858 A321473 A125292

Adjacent sequences: A354145 A354146 A354147 * A354149 A354150 A354151

Keyword

nonn

Author

N. J. A. Sloane, May 22 2022

Status

approved