A251393 Indices of powers of 2 in A098550.

1, 2, 4, 6, 14, 29, 65, 137, 277, 546, 1109, 2208, 4391, 8746, 17404, 34662, 69036, 137604, 274385, 547278, 1091962, 2179171, 4349905, 8684545, 17341369, 34632301, 69172557, 138175535, 276039015, 551501773

Offset: 1

Author

Reinhard Zumkeller, Dec 03 2014

Keywords

• nonn
• more

Links

• Rémy Sigrist, C program

Crossrefs

Subsequence of A251241.

Cf. A098550, A209229, A251394.

Programs

C

// See Links section.

Haskell

a251393 n = a251393_list !! (n-1)
a251393_list = filter ((== 1) . a209229 . a098550) [1..]

Python

from math import gcd
A251393_list, l1, l2, s, p2, b = [1,2], 3, 2, 4, 4, {}
for n in range(4,10**4):
    i = s
    while True:
        if not i in b and gcd(i,l1) == 1 and gcd(i,l2) > 1:
            l2, l1, b[i] = l1, i, 1
            while s in b:
                b.pop(s)
                s += 1
            if i == p2:
                A251393_list.append(n)
                p2 *= 2
            break
        i += 1 # Chai Wah Wu, Dec 03 2014

Formula

A098550(a(n)) = 2^(n-1).

A209229(A098550(a(n))) = 1.

Extensions

a(18)-a(19) from Chai Wah Wu, Dec 03 2014

a(20) from Chai Wah Wu, Dec 04 2014

a(21)-a(22) from Rémy Sigrist, Nov 08 2022

a(23)-a(30) from Jinyuan Wang, Jan 27 2025