From 7dfcc480ba1e19bd3232349fc733caef94034292 Mon Sep 17 00:00:00 2001
From: stainer_t <thomas.stainer@oecd-nea.org>
Date: Mon, 8 Sep 2025 13:48:49 +0200
Subject: Initial commit from Polytechnique Montreal

---
 Dragon/src/SYBWIJ.f | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 Dragon/src/SYBWIJ.f

(limited to 'Dragon/src/SYBWIJ.f')

diff --git a/Dragon/src/SYBWIJ.f b/Dragon/src/SYBWIJ.f
new file mode 100644
index 0000000..a172df4
--- /dev/null
+++ b/Dragon/src/SYBWIJ.f
@@ -0,0 +1,57 @@
+*DECK SYBWIJ
+      SUBROUTINE SYBWIJ (NREG,MAXPTS,SIGW,PIJ)
+*
+*-----------------------------------------------------------------------
+*
+*Purpose:
+* Scattering reduction for collision probabilities.
+*
+*Copyright:
+* Copyright (C) 2002 Ecole Polytechnique de Montreal
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU Lesser General Public
+* License as published by the Free Software Foundation; either
+* version 2.1 of the License, or (at your option) any later version
+*
+*Author(s): A. Hebert
+*
+*Parameters: input
+* NREG    total number of regions.
+* MAXPTS  first dimension of matrix PIJ.
+* SIGW    P0 within-group scattering macroscopic cross sections
+*         ordered by volume.
+*
+*Parameters: input/output
+*  PIJ    reduced collision probability matrix on input and
+*         scattering-reduced collision probability matrix at output.
+*
+*-----------------------------------------------------------------------
+*
+*----
+*  SUBROUTINE ARGUMENTS
+*----
+      INTEGER NREG,MAXPTS
+      REAL SIGW(NREG),PIJ(MAXPTS,NREG)
+*----
+*  LOCAL VARIABLES
+*----
+      REAL, ALLOCATABLE, DIMENSION(:,:) :: WIJ
+*
+      ALLOCATE(WIJ(NREG,2*NREG))
+      DO 20 I=1,NREG
+      DO 10 J=1,NREG
+      WIJ(I,NREG+J)=PIJ(I,J)
+      WIJ(I,J)=-PIJ(I,J)*SIGW(J)
+   10 CONTINUE
+      WIJ(I,I)=1.0+WIJ(I,I)
+   20 CONTINUE
+      CALL ALSB(NREG,NREG,WIJ,IER,NREG)
+      IF(IER.NE.0) CALL XABORT('SYBWIJ: SINGULAR MATRIX.')
+      DO 40 J=1,NREG
+      DO 30 I=1,NREG
+      PIJ(I,J)=WIJ(I,NREG+J)
+   30 CONTINUE
+   40 CONTINUE
+      DEALLOCATE(WIJ)
+      RETURN
+      END
-- 
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