let t1, t2 be Real; :: thesis: for F being real-valued Function st F is t1 -periodic & F is t2 -periodic & t1 + t2 <> 0 holds
F is t1 + t2 -periodic

let F be real-valued Function; :: thesis: ( F is t1 -periodic & F is t2 -periodic & t1 + t2 <> 0 implies F is t1 + t2 -periodic )
assume A1: ( F is t1 -periodic & F is t2 -periodic & t1 + t2 <> 0 ) ; :: thesis: F is t1 + t2 -periodic
for x being Real st x in dom F holds
( x + (t1 + t2) in dom F & x - (t1 + t2) in dom F & F . x = F . (x + (t1 + t2)) )
proof
let x be Real; :: thesis: ( x in dom F implies ( x + (t1 + t2) in dom F & x - (t1 + t2) in dom F & F . x = F . (x + (t1 + t2)) ) )
assume A2: x in dom F ; :: thesis: ( x + (t1 + t2) in dom F & x - (t1 + t2) in dom F & F . x = F . (x + (t1 + t2)) )
then ( x + t1 in dom F & x - t1 in dom F ) by ;
then ( (x + t1) + t2 in dom F & (x - t1) - t2 in dom F & F . (x + t1) = F . ((x + t1) + t2) ) by ;
hence ( x + (t1 + t2) in dom F & x - (t1 + t2) in dom F & F . x = F . (x + (t1 + t2)) ) by A1, A2; :: thesis: verum
end;
hence F is t1 + t2 -periodic by ; :: thesis: verum