From f072be76d5652c502a5bd84cc47cb3dd86c25634 Mon Sep 17 00:00:00 2001
From: Felix Schlepper <felix.schlepper@cern.ch>
Date: Tue, 27 Aug 2024 13:39:59 +0200
Subject: [PATCH] COMMON: Fix flat idx (signed flip)

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>
---
 Common/MathUtils/include/MathUtils/LegendrePols.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Common/MathUtils/include/MathUtils/LegendrePols.h b/Common/MathUtils/include/MathUtils/LegendrePols.h
index dca71c1275962..d5e6a259573f4 100644
--- a/Common/MathUtils/include/MathUtils/LegendrePols.h
+++ b/Common/MathUtils/include/MathUtils/LegendrePols.h
@@ -200,7 +200,7 @@ class Legendre2DPolynominal final : public TNamed,
            boost::math::legendre_p(i - j, y);
   }
 
-  inline int getFlatIdx(int i, int j) const { return i * (i - 1) / 2 + j; }
+  inline int getFlatIdx(int i, int j) const { return i * (i + 1) / 2 + j; }
 
   unsigned int fOrder{0};
   std::vector<double> fParams;