:: deftheorem Def1 defines chi MESFUN12:def 1 :
for A, X being set
for er being ExtReal
for b₄ being Function of X,ExtREAL holds
( b₄ = chi (er,A,X) iff for x being object st x in X holds
( ( x in A implies b₄ . x = er ) & ( not x in A implies b₄ . x = 0 ) ) );