A027172 a(n) = (1/2) * A027170(2n, n).

5, 21, 78, 285, 1048, 3892, 14584, 55053, 209064, 797808, 3056870, 11752676, 45316898, 175175818, 678639148, 2634146413, 10241938408, 39882831448, 155519160718, 607181138848, 2373227900938, 9285480209458, 36364319046898, 142534408279300, 559123836166978

Offset

1,1

Links

Table of n, a(n) for n=1..25.

Formula

a(n) = A027175(n) + 2 = A027171(n)/2.

a(n) = binomial(2*n+2, n+1) + binomial(2*n-1, n-1) - 2. - Christian Krause, May 13 2026

Crossrefs

Cf. A027170, A027171, A027175.

Sequence in context: A268202 A055219 A108863 * A244198 A186244 A255447

Adjacent sequences: A027169 A027170 A027171 * A027173 A027174 A027175

Keyword

nonn,easy

Author

Clark Kimberling

Status

approved