A065494 Number of (binary) bit strings in which no even length block of 0's is followed by an even length block of 1's.

1, 2, 4, 8, 15, 30, 57, 112, 216, 420, 815, 1580, 3069, 5950, 11552, 22408, 43487, 84378, 163725, 317700, 616444, 1196172, 2321007, 4503704, 8738921, 16956954, 32903164, 63845000, 123884479, 240384374, 466440273, 905077080, 1756205088

Offset: 0

Author

Len Smiley, Nov 24 2001

Keywords

• nonn
• easy

Examples

			a(6)=64-7=57 because 000011, 001111, 001100, 001101, 100110, 010011, 110011 are forbidden.
		

Links

• Index entries for linear recurrences with constant coefficients, signature (0,3,2,-1).

Crossrefs

Cf. A061279 (forbids odd block 0's - odd block 1's), A065455, A065495, A065497.

Formula

O.g.f.: (1+x)^2/(1-3x^2-2x^3+x^4)