Number of permutations satisfying i-2<=p(i)<=i+3, i=1..n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =46a(5) =115a(6) =301a(7) =792a(8) =2068a(9) =5380a(10) =14020a(11) =36581a(12) =95413a(13) =248786a(14) =648714a(15) =1691686a(16) =4411530a(17) =11503991a(18) =29998953a(19) =78228640a(20) =203998184a(21) =531969064a(22) =1387222648a(23) =3617479225a(24) =9433351129a(25) =24599481138a(26) =64148406350a(27) =167280683834