# Duality Theorem in Digital Electronics

In digital electronics, there are two basic types of topologies, AND-OR and OR-AND.

OR-AND topology is also known as Product of Sums [ANDing of ORs].

AND-OR topology is also known as Sum of Products [ORing of ANDs]

Using duality theorem, Sum of Products is converted to Product of Sums and vice versa. Each OR gate is changed to AND gate and vice versa, and all 0′s and 1′s are complemented while keeping following points into consideration:

1) A + 1 = 1
2) A + 0 = A
3) A . 1 = A
4) A . 0 = 0
5) A + B = B + A
6) A . B = B . A

Example of Duality Theorem

Let say we have an expression F = ab + bc + ac

To calculate it’s dual, + operator should be changed with . operation and all 0 should be changed to 1. Thus, dual of F is:

Fdual = (a+b) • (b+c) • (a+c)

Now let’s understand how can we obtain F from Fdual:

Fdual = (a+b) • (b+c) • (a+c)
= (ab + ac + b + bc) . (a+c)
= ((ac + b) + b.(a+c)) . (a+c)
= (ac + b).(a+c) + b.(a+c).(a+c)
= ac + ac + ba + bc + b.(a+c)      [A.A = A]
= ac + ba + bc + b.(a+c)         [A+A = A]
= ac + ba + bc + ba + bc
= ac + ba+ bc        [A+A = A]
= ba + bc + ac       [A+B = B+A]
= ab + bc + ac       [AB = BA]
= F

Thus, advantage of duality theorem is that starting from one relation, another relation is obtained which is also a valid boolean identity.

1. I was trying to check duality with F= x+yz+x’z
Fdual= x(y+z)(x’+z) but when trying to put the values in Truth table for both the expressions to check, I am not getting same thing. Please let me know where I am mistaking.

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With Regard
Swiss Bank Corporation