an+1=2an-1
a(n+1)-1=2(an-1)
∴bn=an-1是等比数列
cn=lg(2(an+1 -1)-(an-1))=lg(4(an-1)-(an-1))
=lg3(an-1)
=lg3+lg(an-1)
所以cn应该是等差数列,不是等比数列