a1=λ,依题有a2=(2/3)a1+1-4=(2/3)λ-3,a3=(2/3)a2+2-4=(2/3)a2-2=(2/3)[(2/3)λ-3]-2=(4/9)λ-4.
若a1,a2,a3成等比数列,则有a2的平方=a1*a3,
而题中a2的平方=[(2/3)λ-3]的平方=(4/9)λ的平方-4λ+9,a1*a3=λ[(4/9)λ-4]=(4/9)λ的平方-4λ,a2的平方不等于a1*a3,
故{an}不是等比数列.