在数列{a n }中,如果对任意的n∈N * , a n+2 a n+1 - a n+1 a n =λ (λ为常数),则称数列{a n }为比等差数列,λ称为比公差.则下列命题中真命题的序号是______ ①若数列{F n }满足F 1 =1,F 2 =1,F n =F n-1 +F n-2 (n≥3),则该数列不是比等差数列; ②若数列{a n }满足 a n =(n-1)? 2 n-1 ,则数列{a n }是比等差数列,且比公差λ=2; ③“等差数列是常数列”是“等差数列成为比等差数列”的充分必要条件; ④数列{a n }满足: a 1 = 3 2 ,且 a n = 3n a n-1 2 a n-1 +n-1 (n≥2,n∈N),则此数列的通 a n = n? 3 n 3 n -1 ,且{a n }不是比等差数列.