How we can cheek if this defference equation $-\Delta^2 y(t)=0$ is disconjugate or not? We have this definition: We say that the difference equation $\Delta (p(t-1)\Delta y(t-1))+q(t)y(t)=0$ is "disconjugate" on $[a, b+ 2]=\{a, a+1,...,b+2\}$ provided that no nontrivial solution of $\Delta (p(t-1)\Delta y(t-1))+q(t)y(t)=0$ has two or more generalized zeros on [a, b + 2].
What is a generalized zero?