let a be real number ; :: thesis: for n being Element of NAT st a >= 0 & n >= 1 holds
n -root a >= 0
let n be Element of NAT ; :: thesis: ( a >= 0 & n >= 1 implies n -root a >= 0 )
assume that
A1: a >= 0 and
A2: n >= 1 ; :: thesis: n -root a >= 0

A3: now

per cases ( a > 0 or a = 0 ) by A1;

suppose A4: a > 0 ; :: thesis: n -root a >= 0

A5: n -root a = n -Root a by A1, A2, Def1;
thus n -root a >= 0 by A2, A4, A5, PREPOWER:def 3; :: thesis: verum

end;

suppose A6: a = 0 ; :: thesis: n -root a >= 0

A7: n -root a = n -Root a by A1, A2, Def1
.= 0 by A2, A6, PREPOWER:def 3 ;
thus n -root a >= 0 by A7; :: thesis: verum

end;

end;

end;

thus n -root a >= 0 by A3; :: thesis: verum