Numbers of the form 20*k+7 which are three times a square.

Terms

a(0) =27a(1) =147a(2) =507a(3) =867a(4) =1587a(5) =2187a(6) =3267a(7) =4107a(8) =5547a(9) =6627a(10) =8427a(11) =9747a(12) =11907a(13) =13467a(14) =15987a(15) =17787a(16) =20667a(17) =22707a(18) =25947a(19) =28227a(20) =31827a(21) =34347a(22) =38307a(23) =41067a(24) =45387a(25) =48387a(26) =53067a(27) =56307a(28) =61347a(29) =64827