Euler totient function phi(n): count numbers <= n and prime to n.

Terms

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