let f, g be PartFunc of REAL,REAL; :: thesis:
let a be Real; :: thesis:
assume that
A1: ( right_closed_halfline a c= dom f & right_closed_halfline a c= dom g ) and
A2: f is_+infty_ext_Riemann_integrable_on a and
A3: g is_+infty_ext_Riemann_integrable_on a ; :: thesis: ( f + g is_+infty_ext_Riemann_integrable_on a & infty_ext_right_integral ((f + g),a) = () + () )
consider Intg being PartFunc of REAL,REAL such that
A4: dom Intg = right_closed_halfline a and
A5: for x being Real st x in dom Intg holds
Intg . x = integral (g,a,x) and
A6: Intg is convergent_in+infty and
A7: infty_ext_right_integral (g,a) = lim_in+infty Intg by ;
consider Intf being PartFunc of REAL,REAL such that
A8: dom Intf = right_closed_halfline a and
A9: for x being Real st x in dom Intf holds
Intf . x = integral (f,a,x) and
A10: Intf is convergent_in+infty and
A11: infty_ext_right_integral (f,a) = lim_in+infty Intf by ;
set Intfg = Intf + Intg;
A12: ( dom (Intf + Intg) = right_closed_halfline a & ( for x being Real st x in dom (Intf + Intg) holds
(Intf + Intg) . x = integral ((f + g),a,x) ) )
proof
thus A13: dom (Intf + Intg) = (dom Intf) /\ (dom Intg) by VALUED_1:def 1
.= right_closed_halfline a by A8, A4 ; :: thesis: for x being Real st x in dom (Intf + Intg) holds
(Intf + Intg) . x = integral ((f + g),a,x)

let x be Real; :: thesis: ( x in dom (Intf + Intg) implies (Intf + Intg) . x = integral ((f + g),a,x) )
assume A14: x in dom (Intf + Intg) ; :: thesis: (Intf + Intg) . x = integral ((f + g),a,x)
then A15: a <= x by ;
then A16: ( f is_integrable_on ['a,x'] & f | ['a,x'] is bounded ) by A2;
A17: [.a,x.] c= right_closed_halfline a by XXREAL_1:251;
['a,x'] = [.a,x.] by ;
then A18: ( ['a,x'] c= dom f & ['a,x'] c= dom g ) by ;
A19: ( g is_integrable_on ['a,x'] & g | ['a,x'] is bounded ) by ;
thus (Intf + Intg) . x = (Intf . x) + (Intg . x) by
.= (integral (f,a,x)) + (Intg . x) by A8, A9, A13, A14
.= (integral (f,a,x)) + (integral (g,a,x)) by A4, A5, A13, A14
.= integral ((f + g),a,x) by ; :: thesis: verum
end;
A20: for r being Real ex g being Real st
( r < g & g in dom (Intf + Intg) )
proof
let r be Real; :: thesis: ex g being Real st
( r < g & g in dom (Intf + Intg) )

per cases ( r < a or not r < a ) ;
suppose A21: r < a ; :: thesis: ex g being Real st
( r < g & g in dom (Intf + Intg) )

reconsider g = a as Real ;
take g ; :: thesis: ( r < g & g in dom (Intf + Intg) )
thus ( r < g & g in dom (Intf + Intg) ) by ; :: thesis: verum
end;
suppose A22: not r < a ; :: thesis: ex g being Real st
( r < g & g in dom (Intf + Intg) )

reconsider g = r + 1 as Real ;
take g ; :: thesis: ( r < g & g in dom (Intf + Intg) )
A23: r + 0 < r + 1 by XREAL_1:8;
then a <= g by ;
hence ( r < g & g in dom (Intf + Intg) ) by ; :: thesis: verum
end;
end;
end;
then A24: Intf + Intg is convergent_in+infty by ;
for b being Real st a <= b holds
( f + g is_integrable_on ['a,b'] & (f + g) | ['a,b'] is bounded )
proof
let b be Real; :: thesis: ( a <= b implies ( f + g is_integrable_on ['a,b'] & (f + g) | ['a,b'] is bounded ) )
A25: [.a,b.] c= right_closed_halfline a by XXREAL_1:251;
assume A26: a <= b ; :: thesis: ( f + g is_integrable_on ['a,b'] & (f + g) | ['a,b'] is bounded )
then A27: ( f is_integrable_on ['a,b'] & g is_integrable_on ['a,b'] ) by A2, A3;
['a,b'] = [.a,b.] by ;
then A28: ( ['a,b'] c= dom f & ['a,b'] c= dom g ) by ;
A29: ( f | ['a,b'] is bounded & g | ['a,b'] is bounded ) by A2, A3, A26;
then (f + g) | (['a,b'] /\ ['a,b']) is bounded by RFUNCT_1:83;
hence ( f + g is_integrable_on ['a,b'] & (f + g) | ['a,b'] is bounded ) by ; :: thesis: verum
end;
hence A30: f + g is_+infty_ext_Riemann_integrable_on a by ; :: thesis: infty_ext_right_integral ((f + g),a) = () + ()
lim_in+infty (Intf + Intg) = () + () by ;
hence infty_ext_right_integral ((f + g),a) = () + () by ; :: thesis: verum