let B be AbGroup; :: thesis: multMagma(# the carrier of B,the addF of B #) is commutative Group
set G = multMagma(# the carrier of B,the addF of B #);
A1: for a, b being Element of
for x, y being Element of st a = x & b = y holds
a * b = x + y ;
A2: ( multMagma(# the carrier of B,the addF of B #) is associative & multMagma(# the carrier of B,the addF of B #) is Group-like )

proof

reconsider e = 0. B as Element of ;
thus for a, b, c being Element of holds (a * b) * c = a * (b * c) :: according to GROUP_1:def 4 :: thesis: multMagma(# the carrier of B,the addF of B #) is Group-like

proof

let a, b, c be Element of ; :: thesis: (a * b) * c = a * (b * c)
reconsider x = a, y = b, z = c as Element of ;
thus (a * b) * c = (x + y) + z
.= x + (y + z) by RLVECT_1:def 6
.= a * (b * c) ; :: thesis: verum

end;

take e ; :: according to GROUP_1:def 3 :: thesis: for b₁ being Element of the carrier of multMagma(# the carrier of B,the addF of B #) holds
( b₁ * e = b₁ & e * b₁ = b₁ & ex b₂ being Element of the carrier of multMagma(# the carrier of B,the addF of B #) st
( b₁ * b₂ = e & b₂ * b₁ = e ) )
let a be Element of ; :: thesis: ( a * e = a & e * a = a & ex b₁ being Element of the carrier of multMagma(# the carrier of B,the addF of B #) st
( a * b₁ = e & b₁ * a = e ) )
reconsider x = a as Element of ;
thus a * e = x + (0. B)
.= a by RLVECT_1:10 ; :: thesis: ( e * a = a & ex b₁ being Element of the carrier of multMagma(# the carrier of B,the addF of B #) st
( a * b₁ = e & b₁ * a = e ) )
reconsider b = - x as Element of ;
thus e * a = x + (0. B) by A1
.= a by RLVECT_1:10 ; :: thesis: ex b₁ being Element of the carrier of multMagma(# the carrier of B,the addF of B #) st
( a * b₁ = e & b₁ * a = e )
take b ; :: thesis: ( a * b = e & b * a = e )
thus a * b = x + (- x)
.= e by RLVECT_1:16 ; :: thesis: b * a = e
thus b * a = x + (- x) by A1
.= e by RLVECT_1:16 ; :: thesis: verum

end;

now

let a, b be Element of ; :: thesis: a * b = b * a
reconsider x = a, y = b as Element of ;
thus a * b = y + x by A1
.= b * a ; :: thesis: verum

end;

hence multMagma(# the carrier of B,the addF of B #) is commutative Group by A2, GROUP_1:def 16; :: thesis: verum