O.g.f.: exp( Sum_{n>=1} (sigma(2n)-sigma(n))^2 * x^n/n ).

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =208a(5) =656a(6) =1984a(7) =5632a(8) =15520a(9) =41476a(10) =107312a(11) =271232a(12) =670464a(13) =1622160a(14) =3854208a(15) =9003264a(16) =20696640a(17) =46895248a(18) =104827472a(19) =231353984a(20) =504592448a(21) =1088323584a(22) =2322683072a(23) =4908033280a(24) =10273819136a(25) =21313971876a(26) =43843093488