Numbers k such that k^2 divides 21^k-1.

Terms

a(0) =1a(1) =2a(2) =4a(3) =5a(4) =10a(5) =20a(6) =22a(7) =44a(8) =52a(9) =68a(10) =110a(11) =220a(12) =260a(13) =340a(14) =506a(15) =572a(16) =748a(17) =820a(18) =884a(19) =1012a(20) =2530a(21) =2756a(22) =2860a(23) =3740a(24) =3916a(25) =4108a(26) =4420a(27) =5060a(28) =9020a(29) =9724