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
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.
Formula
a(n)=number of terms of A072685 <= n.
Extensions
a(17) onward corrected by Sean A. Irvine, Oct 20 2024