Numbers k that require four iterations of the sigma function to be >= 2*k.

Terms

a(0) =81a(1) =1681a(2) =3481a(3) =5041a(4) =7921a(5) =17161a(6) =27889a(7) =29929a(8) =83521a(9) =146689a(10) =279841a(11) =491401a(12) =552049a(13) =579121a(14) =635209a(15) =683929a(16) =703921a(17) =707281a(18) =829921a(19) =1190281a(20) =1203409a(21) =1352569a(22) =1481089a(23) =1885129a(24) =2036329a(25) =2211169a(26) =2430481a(27) =2505889a(28) =3279721a(29) =3411409