A073781 Number of 2's in base-3 representation of n-th prime.

1, 0, 1, 1, 1, 0, 2, 1, 2, 1, 0, 0, 1, 1, 2, 3, 2, 2, 1, 3, 2, 3, 1, 2, 1, 2, 1, 3, 0, 1, 1, 2, 2, 1, 2, 2, 2, 1, 2, 2, 3, 2, 2, 1, 3, 1, 2, 3, 3, 2, 4, 4, 4, 2, 1, 2, 3, 0, 1, 1, 0, 2, 1, 2, 2, 3, 1, 0, 2, 2, 1, 2, 1, 1, 1, 2, 2, 2, 3, 1, 2, 2, 4, 1, 2, 2, 3, 3

Offset: 1

Author

Zak Seidov Aug 11 2002

Keywords

• base
• easy
• nonn

Examples

			a(7)=2 as 7th prime 17 is 122 in base-3 representation.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A073779, A073780.

Programs

Mathematica

a2[n_] := Length[Cases[IntegerDigits[Prime[n], 3], 2]]
DigitCount[Prime[Range[90]],3,2] (* Harvey P. Dale, May 11 2016 *)

Formula

a(n) = A081603(A000040(n)). - Michel Marcus, Oct 02 2013