a(n) = (22^n - 1)/21.A218725a(n) = (22^n - 1)/21.Terms a(0) =0a(1) =1a(2) =23a(3) =507a(4) =11155a(5) =245411a(6) =5399043a(7) =118778947a(8) =2613136835a(9) =57489010371External referencesoeis: A218725