From 9675dc765d0af906557d5284819492173eec3735 Mon Sep 17 00:00:00 2001
From: Mike J Innes <mike.j.innes@gmail.com>
Date: Wed, 24 Aug 2016 15:41:30 +0100
Subject: [PATCH] backprop for sigma

---
 src/activation.jl | 2 ++
 src/model.jl      | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/activation.jl b/src/activation.jl
index ab269305..398fe0fe 100644
--- a/src/activation.jl
+++ b/src/activation.jl
@@ -1,3 +1,5 @@
 export σ
 
 σ(x) = 1 ./ (1 .+ exp.(-x))
+
+back!(::typeof(σ), Δ, x) = Δ .* σ(x)./(1.-σ(x))
diff --git a/src/model.jl b/src/model.jl
index 3d5277b3..1591fa3f 100644
--- a/src/model.jl
+++ b/src/model.jl
@@ -5,7 +5,7 @@ export Model, back!, update!, param
 abstract Model
 
 back!(m::Model, ∇) = error("Backprop not implemented for $(typeof(m))")
-update!(m::Model, η) = m
+update!(m, η) = m
 
 # Model parameters