# Zygote hessian

**URL:** <https://discourse.julialang.org/t/zygote-hessian/34998>\
**Category:** New to Julia\
**Tags:** zygote\
**Created:** [February 21, 2020, 11:19pm UTC](https://discourse.julialang.org/t/zygote-hessian/34998 "2020-02-21T23:19:16Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![jacob-roth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jacob-roth/32/1862_2.png) [@jacob-roth](https://discourse.julialang.org/u/jacob-roth)\
**Post date:** [February 21, 2020, 11:19pm UTC](https://discourse.julialang.org/t/zygote-hessian/34998/1 "2020-02-21T23:19:16Z")

</div>

I’m trying to use Zygote to compute a simple Hessian. What am I doing wrong?

```julia
using Zygote
p = 0
function ϕ(x, p)
    return sum( x.^2 )
end
gϕ = x -> Zygote.gradient(xx->ϕ(xx,p), x)
Hϕ = x -> Zygote.hessian(xx->ϕ(xx,p), x)
x0 = [-1.1, 2.1]
gϕ(x0)
Hϕ(x0)

```

The gradient works, but for the Hessian, I get an error:

```julia
ERROR: Need an adjoint for constructor ForwardDiff.Dual{Nothing,Float64,2}. Gradient is of type ForwardDiff.Dual{Nothing,Float64,2}
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] (::Zygote.Jnew{ForwardDiff.Dual{Nothing,Float64,2},Nothing,false})(::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/lib.jl:255
 [3] (::Zygote.var"#287#back#162"{Zygote.Jnew{ForwardDiff.Dual{Nothing,Float64,2},Nothing,false}})(::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/ZygoteRules/Jn4LO/src/adjoint.jl:48
 [4] Dual at /Users/jakeroth/.julia/packages/ForwardDiff/N0wMF/src/dual.jl:19 [inlined]
 [5] (::typeof(∂(ForwardDiff.Dual{Nothing,Float64,2})))(::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/compiler/interface2.jl:0
 [6] literal_pow at /Users/jakeroth/.julia/packages/ForwardDiff/N0wMF/src/dual.jl:55 [inlined]
 [7] (::Zygote.var"#1241#1248")(::typeof(∂(literal_pow)), ::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/broadcast.jl:113
 [8] #3 at ./generator.jl:36 [inlined]
 [9] iterate at ./generator.jl:47 [inlined]
 [10] collect(::Base.Generator{Base.Iterators.Zip{Tuple{Array{typeof(∂(literal_pow)),1},FillArrays.Fill{ForwardDiff.Dual{Nothing,Float64,2},1,Tuple{Base.OneTo{Int64}}}}},Base.var"#3#4"{Zygote.var"#1241#1248"}}) at ./array.jl:622
 [11] map at ./abstractarray.jl:2155 [inlined]
 [12] (::Zygote.var"#1240#1247"{Tuple{Base.RefValue{typeof(^)},Array{ForwardDiff.Dual{Nothing,Float64,2},1},Base.RefValue{Val{2}}},Val{4},Array{typeof(∂(literal_pow)),1}})(::FillArrays.Fill{ForwardDiff.Dual{Nothing,Float64,2},1,Tuple{Base.OneTo{Int64}}}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/broadcast.jl:113
 [13] #3186#back at /Users/jakeroth/.julia/packages/ZygoteRules/Jn4LO/src/adjoint.jl:48 [inlined]
 [14] #143 at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/lib.jl:132 [inlined]
 [15] #252#back at /Users/jakeroth/.julia/packages/ZygoteRules/Jn4LO/src/adjoint.jl:48 [inlined]
 [16] broadcasted at ./broadcast.jl:1237 [inlined]
 [17] ϕ at ./REPL[68]:2 [inlined]
 [18] (::typeof(∂(ϕ)))(::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/compiler/interface2.jl:0
 [19] #90 at ./REPL[70]:1 [inlined]
 [20] (::Zygote.var"#32#33"{typeof(∂(#90))})(::ForwardDiff.Dual{Nothing,Float64,2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/compiler/interface.jl:38
 [21] gradient(::Function, ::Array{ForwardDiff.Dual{Nothing,Float64,2},1}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/compiler/interface.jl:47
 [22] (::Zygote.var"#1303#1304"{var"#90#92"})(::Array{ForwardDiff.Dual{Nothing,Float64,2},1}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/utils.jl:81
 [23] forward_jacobian(::Zygote.var"#1303#1304"{var"#90#92"}, ::Array{Float64,1}, ::Val{2}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/forward.jl:23
 [24] forward_jacobian(::Function, ::Array{Float64,1}) at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/forward.jl:38
 [25] hessian at /Users/jakeroth/.julia/packages/Zygote/bdE6T/src/lib/utils.jl:81 [inlined]
 [26] (::var"#89#91")(::Array{Float64,1}) at ./REPL[70]:1
 [27] top-level scope at REPL[73]:100:

```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [February 21, 2020, 11:49pm UTC](https://discourse.julialang.org/t/zygote-hessian/34998/2 "2020-02-21T23:49:52Z")

</div>

[https://github.com/FluxML/Zygote.jl/pull/510](https://github.com/FluxML/Zygote.jl/pull/510)

---

<div class="post-metadata">

**Author:** ![e3c6](https://avatars.discourse-cdn.com/v4/letter/e/e79b87/32.png) [@e3c6](https://discourse.julialang.org/u/e3c6)\
**Post date:** [May 30, 2020, 10:40am UTC](https://discourse.julialang.org/t/zygote-hessian/34998/3 "2020-05-30T10:40:52Z")

</div>

For what it is worth, this example now runs fine in Zygote.

---

<div class="post-metadata">

**Author:** ![Yansf677](https://avatars.discourse-cdn.com/v4/letter/y/90ced4/32.png) [@Yansf677](https://discourse.julialang.org/u/Yansf677)\
**Post date:** [August 10, 2020, 9:08am UTC](https://discourse.julialang.org/t/zygote-hessian/34998/4 "2020-08-10T09:08:11Z")

</div>

when I calculate the gradient of a function consiting of LinearAlgebra.diagm, I encounter similar error:  
`Need an adjoint for constructor Pair{Int64,Array{Float64,1}}`  
How can I solve this problem?