A108853 Indices of prime Knuth numbers; that is, integers n such that the n-th Knuth number is prime. Indices of primes in A007448.

1, 2, 4, 5, 6, 10, 11, 12, 15, 16, 17, 18, 28, 29, 30, 40, 41, 42, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 121, 122, 123, 124, 125, 126, 136, 137, 138, 159, 160, 161, 162, 190, 191, 192, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213

Offset: 1

Author

Ryan Propper, Jul 11 2005

Keywords

• easy
• nonn

Examples

			10 is a term because the 10th Knuth number, 13, is prime.
		

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..10000
• Eric Weisstein's World of Mathematics, Knuth Number.

Crossrefs

Cf. A007448.

Programs

Python

from sympy import isprime, primepi
from itertools import count, islice
def agen():
    A007448n, A007448lst = 1, [1]
    for n in count(0):
        if isprime(A007448n):
            yield n
        A007448n = 1 + min(2*A007448lst[n//2], 3*A007448lst[n//3])
        A007448lst.append(A007448n)
print(list(islice(agen(), 61))) # Michael S. Branicky, Jul 15 2022

Extensions

Extended by Ray Chandler, Jul 24 2005