a(1) = 15; for n > 1, a(n)^2 is the smallest square that begins with a(n-1) in base 6.

Terms

a(0) =15a(1) =57a(2) =111a(3) =155a(4) =183a(5) =199a(6) =508a(7) =812a(8) =171a(9) =471a(10) =319a(11) =643a(12) =913a(13) =1088a(14) =2909a(15) =11650a(16) =9518a(17) =21074a(18) =31357a(19) =93691a(20) =396693a(21) =55540a(22) =50905a(23) =119374a(24) =182804a(25) =226216a(26) =251647a(27) =265415a(28) =111280a(29) =72055