Number of alternating separable permutations.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =20a(5) =48a(6) =132a(7) =344a(8) =996a(9) =2720a(10) =8132a(11) =22888a(12) =69940a(13) =201040a(14) =624132a(15) =1822136a(16) =5725124a(17) =16915008a(18) =53648772a(19) =160012232a(20) =511360340a(21) =1536928624a(22) =4942300804a(23) =14949122328a(24) =48322714020a(25) =146946942688a(26) =477105960772