A function can be recognized that weather it is self dual or not by following two conditions:-
1> IT should have 2^(n-1) min terms ..................... where n = no. of terms in the fn
eg. f(x,y,z)= summation (1,2,4,7) .........................here n = 3................as no. of terms are x y z so
2^(3-1)=4 which is our no. minterms in fn
2> If mi is the corresponding mean term in the fn
then m((2^n-1)-i) should not belong to our fn
eg. in above example f(x,y,z)=summation(1,2,4,7)
m1 is present and our i=1 and from above n =3
so by rule 2
m((2^3-1)-1)=m6 which do not belong to our f ......... similarly check for 2,4,7
if any os the above rule fails then it is not self dual
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hope u got this