A236866 Positions of primes in A007775 (numbers not divisible by 2, 3 or 5).

2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 22, 23, 24, 26, 27, 28, 29, 30, 31, 34, 35, 37, 38, 40, 41, 42, 44, 45, 47, 48, 49, 51, 52, 53, 54, 57, 60, 61, 62, 63, 64, 65, 67, 69, 71, 72, 73, 74, 75, 76, 79, 82, 83, 84, 85, 89, 90, 93, 94, 95

Offset: 1

Author

Alex Ratushnyak, Jan 31 2014

Keywords

• nonn
• easy

Comments

From Antti Karttunen, Feb 01 2014: (Start)

Positions of primes among natural numbers coprime to 30.

Term 1 is missing from the sequence, because A007775(1)=1 is not considered a prime, terms 2 - 13 are all present, and 14 is the next term missing from here, as A007775(14)=49 is the first composite in that sequence.

(End)

Links

• Antti Karttunen, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A007775, A181709.

Programs

Python

from sympy import isprime
i=0
for n in range(1000):
    if n%2 and n%3 and n%5:
        i+=1   # A007775(i)=n
        if isprime(n):  print(i, end=', ')

Scheme

;; With Antti Karttunen's IntSeq-library.
(define A236866 (MATCHING-POS 1 1 (lambda (n) (prime? (A007775 n)))))
;; Where a slow version of A007775 can be defined for example like this,
(define A007775 (MATCHING-POS 1 1 (lambda (n) (= 1 (gcd n 30)))))
;; from Antti Karttunen, Feb 01 2014