From 1b5e416fed67965024296ae7262024bc14adf2d3 Mon Sep 17 00:00:00 2001
From: stertooy <5571903+stertooy@users.noreply.github.com>
Date: Sun, 29 Mar 2026 23:55:34 +0200
Subject: [PATCH] Use TryNextMethod instead of Error in Intersection2 if at
 least one group is finite

---
 gap/pcpgrp/inters.gi |  3 +++
 tst/inters.tst       | 14 ++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/gap/pcpgrp/inters.gi b/gap/pcpgrp/inters.gi
index 834cf9c..42cfeba 100644
--- a/gap/pcpgrp/inters.gi
+++ b/gap/pcpgrp/inters.gi
@@ -151,5 +151,8 @@ function( U, V )
         return NormalIntersection( V, U );
     fi;
 
+    if IsFinite( U ) or IsFinite( V ) then
+        TryNextMethod();
+    fi;
     Error("sorry: intersection for non-normal groups not yet installed");
 end );
diff --git a/tst/inters.tst b/tst/inters.tst
index 3528a2c..691261b 100644
--- a/tst/inters.tst
+++ b/tst/inters.tst
@@ -94,7 +94,7 @@ Pcp-group with orders [ 5, 3, 3, 3, 2 ]
 gap> H2 := Subgroup(H,[h[1]*h[7], h[4]*h[5]]);
 Pcp-group with orders [ 3, 3, 3, 3 ]
 gap> J := Intersection(H1,H2);
-Error, sorry: intersection for non-normal groups not yet installed
+Pcp-group with orders [ 3, 3, 3 ]
 gap> G1 := PreImagesNC(iso,H1);
 Group([ f2, f3*f4, f6 ])
 gap> G2 := PreImagesNC(iso,H2);
@@ -114,7 +114,7 @@ Pcp-group with orders [ 2, 3, 3, 3 ]
 gap> H2 := Subgroup(H,[h[1]*h[7], h[4]*h[5]^2]);
 Pcp-group with orders [ 2, 3, 3, 3 ]
 gap> Intersection(H1,H2);
-Error, sorry: intersection for non-normal groups not yet installed
+Pcp-group with orders [ 3, 3 ]
 gap> G1 := PreImagesNC(iso,H1);
 Group([ f2, f3*f4, f6^2 ])
 gap> G2 := PreImagesNC(iso,H2);
@@ -124,5 +124,15 @@ Group([ f6^2, f7^2 ])
 gap> Image(iso,I);
 Pcp-group with orders [ 3, 3 ]
 
+# finite - infinite combination example where the intersection isn't impl. when represented as a pcp-group (non-normalizing case)
+gap> G := DirectProduct(ExamplesOfSomePcpGroups(8), PcGroupToPcpGroup(PcGroupCode(2835879971,72)));;
+gap> g := GeneratorsOfGroup(G);;
+gap> H1 := Subgroup(G,[g[6]*g[9]^2]);
+Pcp-group with orders [ 2, 2 ]
+gap> H2 := Subgroup(G,[g[6],g[2]*g[7]]);
+Pcp-group with orders [ 0, 2, 2 ]
+gap> Intersection(H1,H2);
+Pcp-group with orders [ 2 ]
+
 #
 gap> STOP_TEST( "inters.tst", 10000000);