A072688 Of the first n integers (inclusive), how many of them have a prime number of letters in their English names?

1, 2, 3, 3, 3, 4, 5, 6, 6, 7, 7, 7, 7, 7, 8, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 11, 12, 12, 12, 12, 12, 13, 13, 13, 13, 14, 15, 15, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 19, 19

Offset: 1

Author

Mark Hudson (mrmarkhudson(AT)hotmail.com), Jul 02 2002

Keywords

• easy
• nonn
• word

Examples

			Sequence A072685 begins: 1, 2, 3, 6, 7, 8, 10 so there are 3 terms up to and including n=4, 3 terms up to and including n=5, 6 terms up to and including n=9. Therefore a(4)=3, a(5)=3, a(9)=6.
		

Crossrefs

Cf. A072685, A072687.

Formula

a(n)=number of terms of A072685 <= n.

Extensions

a(17) onward corrected by Sean A. Irvine, Oct 20 2024