let G1 be non trivial _Graph; :: thesis: for v being Vertex of G1
for G2 being removeVertex of G1,v st G2 is connected & ex e being set st
( e in v .edgesInOut() & not e Joins v,v,G1 ) holds
G1 is connected

let v be Vertex of G1; :: thesis: for G2 being removeVertex of G1,v st G2 is connected & ex e being set st
( e in v .edgesInOut() & not e Joins v,v,G1 ) holds
G1 is connected

let G2 be removeVertex of G1,v; :: thesis: ( G2 is connected & ex e being set st
( e in v .edgesInOut() & not e Joins v,v,G1 ) implies G1 is connected )

assume that
A1: G2 is connected and
A2: ex e being set st
( e in v .edgesInOut() & not e Joins v,v,G1 ) ; :: thesis: G1 is connected
A3: now :: thesis: for x being Vertex of G1 st x <> v holds
x in the_Vertices_of G2
end;
consider e being set such that
A4: e in v .edgesInOut() and
A5: not e Joins v,v,G1 by A2;
set v3 = v .adj e;
v <> v .adj e by ;
then reconsider v39 = v .adj e as Vertex of G2 by A3;
A6: e Joins v,v .adj e,G1 by ;
then A7: e Joins v .adj e,v,G1 by GLIB_000:14;
now :: thesis: for v1, v2 being Vertex of G1 ex W being Walk of G1 st W is_Walk_from v1,v2
let v1, v2 be Vertex of G1; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
now :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
per cases ( v1 <> v or v1 = v ) ;
suppose v1 <> v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
then reconsider v19 = v1 as Vertex of G2 by A3;
now :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
per cases ( v2 <> v or v2 = v ) ;
suppose v2 <> v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
then reconsider v29 = v2 as Vertex of G2 by A3;
consider W9 being Walk of G2 such that
A8: W9 is_Walk_from v19,v29 by A1;
reconsider W = W9 as Walk of G1 by GLIB_001:167;
W is_Walk_from v1,v2 by ;
hence ex W being Walk of G1 st W is_Walk_from v1,v2 ; :: thesis: verum
end;
suppose A9: v2 = v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
consider W9 being Walk of G2 such that
A10: W9 is_Walk_from v19,v39 by A1;
reconsider W = W9 as Walk of G1 by GLIB_001:167;
W is_Walk_from v1,v .adj e by ;
then ( W .first() = v1 & W .last() = v .adj e ) by GLIB_001:def 23;
then W .addEdge e is_Walk_from v1,v2 by ;
hence ex W being Walk of G1 st W is_Walk_from v1,v2 ; :: thesis: verum
end;
end;
end;
hence ex W being Walk of G1 st W is_Walk_from v1,v2 ; :: thesis: verum
end;
suppose A11: v1 = v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
now :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
per cases ( v2 <> v or v2 = v ) ;
suppose v2 <> v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
then reconsider v29 = v2 as Vertex of G2 by A3;
set W1 = G1 .walkOf (v1,e,(v .adj e));
consider W29 being Walk of G2 such that
A12: W29 is_Walk_from v39,v29 by A1;
reconsider W2 = W29 as Walk of G1 by GLIB_001:167;
A13: W2 is_Walk_from v .adj e,v2 by ;
take W = (G1 .walkOf (v1,e,(v .adj e))) .append W2; :: thesis: W is_Walk_from v1,v2
G1 .walkOf (v1,e,(v .adj e)) is_Walk_from v1,v .adj e by ;
hence W is_Walk_from v1,v2 by ; :: thesis: verum
end;
suppose A14: v2 = v ; :: thesis: ex W being Walk of G1 st W is_Walk_from v1,v2
take W = G1 .walkOf v; :: thesis: W is_Walk_from v1,v2
thus W is_Walk_from v1,v2 by ; :: thesis: verum
end;
end;
end;
hence ex W being Walk of G1 st W is_Walk_from v1,v2 ; :: thesis: verum
end;
end;
end;
hence ex W being Walk of G1 st W is_Walk_from v1,v2 ; :: thesis: verum
end;
hence G1 is connected ; :: thesis: verum