Central terms of triangle A131909.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =26a(5) =79a(6) =274a(7) =964a(8) =3496a(9) =12664a(10) =46157a(11) =168240a(12) =615173a(13) =2256669a(14) =8321911a(15) =30871862a(16) =115280183a(17) =433247309a(18) =1638036186a(19) =6225871432a(20) =23769249188a(21) =91077899373a(22) =350002189936