Third column of PE^2.

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =36a(5) =220a(6) =1410a(7) =9534a(8) =68040a(9) =511704a(10) =4046310a(11) =33560010a(12) =291244668a(13) =2638581972a(14) =24901833866a(15) =244333004790