for f being eventually-nonnegative Real_Sequence holds Big_Theta f = { t where t is Element of Funcs (NAT,REAL) : ex c, d being Real ex N being Element of NAT st
( c > 0 & d > 0 & ( for n being Element of NAT st n >= N holds
( d * (f . n) <= t . n & t . n <= c * (f . n) ) ) ) }