Fourth column sequence of triangle A056588.

Terms

a(0) =1a(1) =7a(2) =53a(3) =318a(4) =1784a(5) =9288a(6) =46233a(7) =221859a(8) =1036585a(9) =4742650a(10) =21350264a(11) =94895016a(12) =417562385a(13) =1822824255a(14) =7907350365a(15) =34130697462a(16) =146735898760a(17) =628870859320