Number of idempotent order-preserving partial transformations (of an n-element chain).

A112091

Number of idempotent order-preserving partial transformations (of an n-element chain).

Terms

    a(0) =1a(1) =2a(2) =6a(3) =21a(4) =76a(5) =276a(6) =1001a(7) =3626a(8) =13126a(9) =47501a(10) =171876a(11) =621876a(12) =2250001a(13) =8140626a(14) =29453126a(15) =106562501a(16) =385546876a(17) =1394921876a(18) =5046875001a(19) =18259765626a(20) =66064453126a(21) =239023437501a(22) =864794921876

External references