From 9b439298d08db5ea519408a9ae3f60d8d9029cb6 Mon Sep 17 00:00:00 2001
From: Timothy Nunn <timothy.nunn@ukaea.uk>
Date: Fri, 11 Apr 2025 09:08:22 +0000
Subject: [PATCH] Correct BFGS update

---
 docs/source/vmcon.rst | 2 +-
 src/pyvmcon/vmcon.py  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/docs/source/vmcon.rst b/docs/source/vmcon.rst
index 4e42571..1267aa1 100644
--- a/docs/source/vmcon.rst
+++ b/docs/source/vmcon.rst
@@ -164,7 +164,7 @@ The definition of :math:`\vec{\eta}` ensures :math:`\mathbf{B}` remains positive
 We can then perform the BFGS update:
 
 .. math::
-    \mathbf{B_{NEW}} = \mathbf{B} - \frac{\mathbf{B}\vec{\xi}\vec{\xi}^T\mathbf{B}}{\vec{\xi}^T\mathbf{B}\vec{\xi}} + \frac{ \vec{\eta} \vec{\eta}^T}{\vec{\xi}^T\vec{\eta}}
+    \mathbf{B_{NEW}} = \mathbf{B} - \frac{\mathbf{B}\vec{\xi}\vec{\xi}^T\mathbf{B}^T}{\vec{\xi}^T\mathbf{B}\vec{\xi}} + \frac{ \vec{\eta} \vec{\eta}^T}{\vec{\xi}^T\vec{\eta}}
 
 
 The VMCON Algorithm
diff --git a/src/pyvmcon/vmcon.py b/src/pyvmcon/vmcon.py
index 08b3492..46a60a7 100644
--- a/src/pyvmcon/vmcon.py
+++ b/src/pyvmcon/vmcon.py
@@ -544,7 +544,7 @@ def calculate_new_B(
 
     # eqn 8
     B_ksi = B @ ksi
-    B += (gamma @ gamma.T) / (ksi.T @ gamma) - ((B_ksi @ ksi.T @ B) / (ksi.T @ B_ksi))
+    B += (gamma @ gamma.T) / (ksi.T @ gamma) - ((B_ksi @ ksi.T @ B.T) / (ksi.T @ B_ksi))
 
     return B