Number of compositions of n with exactly 3 transitions between different parts.

Terms

a(0) =2a(1) =10a(2) =36a(3) =86a(4) =200a(5) =374a(6) =680a(7) =1122a(8) =1796a(9) =2694a(10) =3954a(11) =5600a(12) =7752a(13) =10448a(14) =13798a(15) =18072a(16) =23032a(17) =29218a(18) =36390a(19) =45044a(20) =54870a(21) =66852a(22) =79790a(23) =95550a(24) =112662a(25) =132938a(26) =154752a(27) =180614a(28) =207764a(29) =239784