2*a(n)^2 + 7 is a square.

Terms

a(0) =1a(1) =3a(2) =9a(3) =19a(4) =53a(5) =111a(6) =309a(7) =647a(8) =1801a(9) =3771a(10) =10497a(11) =21979a(12) =61181a(13) =128103a(14) =356589a(15) =746639a(16) =2078353a(17) =4351731a(18) =12113529a(19) =25363747a(20) =70602821a(21) =147830751a(22) =411503397a(23) =861620759a(24) =2398417561a(25) =5021893803a(26) =13979001969a(27) =29269742059a(28) =81475594253