let G be strict cyclic Group; :: thesis: for H being strict Subgroup of G holds H is cyclic Group
let H be strict Subgroup of G; :: thesis: H is cyclic Group
per cases ( H = (1). G or H <> (1). G ) ;
suppose H = (1). G ; :: thesis: H is cyclic Group
end;
suppose A1: H <> (1). G ; :: thesis: H is cyclic Group
consider b being Element of G such that
A2: multMagma(# the carrier of G, the multF of G #) = gr {b} by GR_CY_1:def 7;
ex m being Nat st
( 0 < m & b |^ m in H & ( for j being Nat st 0 < j & b |^ j in H holds
m <= j ) )
proof
defpred S1[ Nat] means ( 0 < \$1 & b |^ \$1 in H );
ex k being Nat st S1[k] by A1, A2, Th24;
then A3: ex k being Nat st S1[k] ;
ex m being Nat st
( S1[m] & ( for j being Nat st S1[j] holds
m <= j ) ) from hence ex m being Nat st
( 0 < m & b |^ m in H & ( for j being Nat st 0 < j & b |^ j in H holds
m <= j ) ) ; :: thesis: verum
end;
then consider m being Nat such that
A4: 0 < m and
A5: b |^ m in H and
A6: for j being Nat st 0 < j & b |^ j in H holds
m <= j ;
set c = b |^ m;
reconsider c = b |^ m as Element of H by A5;
for a being Element of H ex i being Integer st a = c |^ i
proof
let a be Element of H; :: thesis: ex i being Integer st a = c |^ i
ex t being Integer st
( b |^ t in H & b |^ t = a )
proof
A7: a in G by GROUP_2:41;
A8: a in gr {b} by ;
a is Element of G by A7;
then consider j1 being Integer such that
A9: a = b |^ j1 by ;
b |^ j1 in H by A9;
hence ex t being Integer st
( b |^ t in H & b |^ t = a ) by A9; :: thesis: verum
end;
then consider t being Integer such that
A10: b |^ t in H and
A11: b |^ t = a ;
ex r, s being Integer st
( 0 <= s & s < m & t = (m * r) + s )
proof
take t div m ; :: thesis: ex s being Integer st
( 0 <= s & s < m & t = (m * (t div m)) + s )

take t mod m ; :: thesis: ( 0 <= t mod m & t mod m < m & t = (m * (t div m)) + (t mod m) )
thus ( 0 <= t mod m & t mod m < m & t = (m * (t div m)) + (t mod m) ) by ; :: thesis: verum
end;
then consider r, s being Integer such that
A12: 0 <= s and
A13: s < m and
A14: t = (m * r) + s ;
reconsider s = s as Element of NAT by ;
A15: b |^ t = (b |^ (m * r)) * (b |^ s) by
.= ((b |^ m) |^ r) * (b |^ s) by GROUP_1:35 ;
A16: (b |^ m) |^ r in H by ;
b |^ s in H
proof
set u = b |^ t;
set v = (b |^ m) |^ r;
A17: (((b |^ m) |^ r) ") * (b |^ t) = ((((b |^ m) |^ r) ") * ((b |^ m) |^ r)) * (b |^ s) by
.= (1_ G) * (b |^ s) by GROUP_1:def 5
.= b |^ s by GROUP_1:def 4 ;
((b |^ m) |^ r) " in H by ;
hence b |^ s in H by ; :: thesis: verum
end;
then s = 0 by ;
then b |^ s = 1_ G by GROUP_1:25;
then A18: b |^ t = (b |^ m) |^ r by ;
(b |^ m) |^ r = c |^ r by GROUP_4:2;
hence ex i being Integer st a = c |^ i by ; :: thesis: verum
end;
then H = gr {c} by GR_CY_2:5;
hence H is cyclic Group by GR_CY_2:4; :: thesis: verum
end;
end;