let f be PartFunc of REAL,REAL; :: thesis:
let c be Real; :: thesis:
assume that
A1: left_closed_halfline c c= dom f and
A2: f is_-infty_improper_integrable_on c and
A3: improper_integral_-infty (f,c) = +infty ; :: thesis:
let b be Real; :: thesis:
assume A4: b <= c ; :: thesis:
consider I being PartFunc of REAL,REAL such that
A5: dom I = ].-infty,c.] and
A6: for x being Real st x in dom I holds
I . x = integral (f,x,c) and
A7: ( I is convergent_in-infty or I is divergent_in-infty_to+infty or I is divergent_in-infty_to-infty ) by A2;
improper_integral_-infty (f,c) <> infty_ext_left_integral (f,c) by A3;
then A8: not f is_-infty_ext_Riemann_integrable_on c by A2, Th22;
-infty < b by XREAL_0:def 1, XXREAL_0:12;
then reconsider LB = ].-infty,b.] as non empty Subset of REAL by XXREAL_1:2;
deffunc H₁( Element of LB) -> Element of REAL = In ((integral (f,$1,b)),REAL);
consider Intf being Function of LB,REAL such that
A9: for x being Element of LB holds Intf . x = H₁(x) from FUNCT_2:sch 4();
A10: dom Intf = LB by FUNCT_2:def 1;
then reconsider Intf = Intf as PartFunc of REAL,REAL by RELSET_1:5;
A11: for x being Real st x in dom Intf holds
Intf . x = integral (f,x,b)

let x be Real; :: thesis:
assume x in dom Intf ; :: thesis:
then Intf . x = In ((integral (f,x,b)),REAL) by A9, A10;
hence Intf . x = integral (f,x,b) ; :: thesis:

end;

A12: for r being Real ex g being Real st
( g < r & g in dom Intf )

let r be Real; :: thesis:
set R = min (b,r);
consider g being Real such that
A13: ( g < min (b,r) & g in dom I ) by A8, A3, A2, A5, Def3, INTEGR10:def 6, LIMFUNC1:48;
A14: ( min (b,r) <= b & min (b,r) <= r ) by XXREAL_0:17;
then A15: ( g < b & g < r ) by A13, XXREAL_0:2;
g in dom Intf by A10, A14, A13, XXREAL_0:2, XXREAL_1:234;
hence ex g being Real st
( g < r & g in dom Intf ) by A15; :: thesis:

end;

A16: for g1 being Real ex r being Real st
for r1 being Real st r1 < r & r1 in dom Intf holds
g1 < Intf . r1

let g1 be Real; :: thesis:
consider r being Real such that
A17: for r1 being Real st r1 < r & r1 in dom I holds
g1 + (integral (f,b,c)) < I . r1 by A8, A7, A3, A2, A5, A6, Def3, INTEGR10:def 6, LIMFUNC1:48;
set R = min (b,r);
take min (b,r) ; :: thesis:
thus for r1 being Real st r1 < min (b,r) & r1 in dom Intf holds
g1 < Intf . r1 :: thesis:

let r1 be Real; :: thesis:
assume A18: ( r1 < min (b,r) & r1 in dom Intf ) ; :: thesis:
( min (b,r) <= b & min (b,r) <= r ) by XXREAL_0:17;
then A19: ( r1 < b & r1 < r ) by A18, XXREAL_0:2;
then A20: r1 < c by A4, XXREAL_0:2;
r1 in dom I by A5, A19, A4, XXREAL_0:2, XXREAL_1:234;
then g1 + (integral (f,b,c)) < I . r1 by A17, A19;
then A21: g1 + (integral (f,b,c)) < integral (f,r1,c) by A6, A20, A5, XXREAL_1:234;
A22: ( f is_integrable_on ['r1,c'] & f | ['r1,c'] is bounded ) by A20, A2;
A23: ['r1,c'] = [.r1,c.] by A19, A4, XXREAL_0:2, INTEGRA5:def 3;
then ['r1,c'] c= ].-infty,c.] by XXREAL_1:265;
then A24: ['r1,c'] c= dom f by A1;
b in ['r1,c'] by A23, A19, A4, XXREAL_1:1;
then integral (f,r1,c) = (integral (f,r1,b)) + (integral (f,b,c)) by A20, A22, A24, INTEGRA6:17;
then g1 < integral (f,r1,b) by A21, XREAL_1:6;
hence g1 < Intf . r1 by A11, A18; :: thesis:

end;

end;

hence A25: f is_-infty_improper_integrable_on b by A10, A11, A1, A2, A4, Lm2, A12, LIMFUNC1:48; :: thesis:
Intf is divergent_in-infty_to+infty by A12, A16, LIMFUNC1:48;
hence improper_integral_-infty (f,b) = +infty by A10, A11, A25, Def3; :: thesis: