Sum of Euler totient function between successive powers of two.

Terms

a(0) =1a(1) =3a(2) =14a(3) =54a(4) =236a(5) =920a(6) =3730a(7) =14862a(8) =59776a(9) =238856a(10) =956110a(11) =3823410a(12) =15299542a(13) =61193632a(14) =244779854a(15) =979111158a(16) =3916558030a(17) =15666093384a(18) =62664665094a(19) =250658276292