Doubly square pyramidal numbers.

Terms

a(0) =0a(1) =1a(2) =55a(3) =1015a(4) =9455a(5) =56980a(6) =255346a(7) =924490a(8) =2850730a(9) =7757035a(10) =19096385a(11) =43312841a(12) =91753025a(13) =183453270a(14) =349074740a(15) =636310340a(16) =1117143236a(17) =1897397285a(18) =3129084635a(19) =5026125195a(20) =7884086595a(21) =12104671656a(22) =18225763270a(23) =26957923950a(24) =39228339150a(25) =56233289775a(26) =79500340101a(27) =110961532605