Triangle of coefficients in expansion of (1+3*x)^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =1a(4) =6a(5) =9a(6) =1a(7) =9a(8) =27a(9) =27a(10) =1a(11) =12a(12) =54a(13) =108a(14) =81a(15) =1a(16) =15a(17) =90a(18) =270a(19) =405a(20) =243a(21) =1a(22) =18a(23) =135a(24) =540a(25) =1215a(26) =1458a(27) =729a(28) =1a(29) =21