Odd elements in Pascal's triangle.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =3a(7) =3a(8) =1a(9) =1a(10) =1a(11) =1a(12) =5a(13) =5a(14) =1a(15) =1a(16) =15a(17) =15a(18) =1a(19) =1a(20) =7a(21) =21a(22) =35a(23) =35a(24) =21a(25) =7a(26) =1a(27) =1a(28) =1a(29) =1