# Differential equations: Vern family solver, lazy option question
**URL:** https://discourse.julialang.org/t/differential-equations-vern-family-solver-lazy-option-question/39865
**Category:** General Usage
**Created:** [May 21, 2020, 2:21am UTC](https://discourse.julialang.org/t/differential-equations-vern-family-solver-lazy-option-question/39865 "2020-05-21T02:21:03Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Egwene\_al\_Vere](https://discourse.julialang.org/u/Egwene_al_Vere)
#### Post date: [May 21, 2020, 2:21am UTC](https://discourse.julialang.org/t/differential-equations-vern-family-solver-lazy-option-question/39865/1 "2020-05-21T02:21:03Z")
For the Verner family solver of `DifferentialEquations.jl`, `Vern6()` through `Vern9()`, it is documented that the interpolation is “lazy” and
> These methods when lazy assume that the parameter vector `p` will be unchanged between the moment of the interval solving and the interpolation. If `p` is changed in a ContinuousCallback, or in a DiscreteCallback and the continuous solution is used after the full solution, then set `lazy=false` .
> [link](https://docs.sciml.ai/stable/solvers/ode_solve/#Full-List-of-Methods-1)
In my code, I used p to allocate the temporary matrix for things like `mul!` e.g.
```julia
hdim = 10
p = (
x=zeros(Complex{Float64}, hdim, hdim),
y=zeros(Complex{Float64}, hdim, hdim)
)
```
and in the ode problem function, `p.x`, `p.y`, etc is used to hold temporary allocations, pseudo-code:
```julia
function f!(du,u,p,t)
...
mul!(p.x, a, b)
...
...
du .= p.x .+ p.y
end
```
so p changes during calculation (different p before and after `f!` gets called, and different p at different times). However, different input value of p does not change the calculation result since it’s always overwritten inside `f!()` with e.g. `mul!(p.x, a, b)`. In this case, is the `lazy=false` option needed? Thanks a lot!
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [May 21, 2020, 2:40am UTC](https://discourse.julialang.org/t/differential-equations-vern-family-solver-lazy-option-question/39865/2 "2020-05-21T02:40:53Z")
> [@Egwene\_al\_Vere](#):
>
> In this case, is the `lazy=false` option needed?
Nope, you’re fine.
---
### Author:  [@Egwene\_al\_Vere](https://discourse.julialang.org/u/Egwene_al_Vere)
#### Post date: [May 21, 2020, 2:44am UTC](https://discourse.julialang.org/t/differential-equations-vern-family-solver-lazy-option-question/39865/3 "2020-05-21T02:44:43Z")
I thought so. Thanks a lot!