İlk olarak \tau_1\star\tau_2\in \mathcal{T} olduğunu gösterelim.
U\in\tau_1\Rightarrow \pi_1^{-1}[U]=\{(x,y)|\pi_1(x,y)\in U\}=\{(x,y)|x\in U\}=U\times Y\in \tau_1\star\tau_2\Big{/} \pi_1, \,\ (\tau_1\star\tau_2\mbox{ - }\tau_1) \text{ sürekli}\ldots (1)
V\in\tau_2\Rightarrow \pi_2^{-1}[V]=\{(x,y)|\pi_2(x,y)\in V\}=\{(x,y)|y\in V\}=X\times V\in \tau_1\star\tau_2\Big{/} \pi_2, \,\ (\tau_1\star\tau_2\mbox{ - }\tau_2) \text{ sürekli}\ldots (2)
(1),(2)\Rightarrow \tau_1\star\tau_2\in \mathcal{T}\ldots (3)
Şimdi de \tau\in \mathcal{T}\Rightarrow \tau_1\star\tau_2\subseteq\tau olduğunu gösterirsek ispat biter.
\tau\in\mathcal{T}
\Rightarrow
(\pi_1:X\times Y\to X, \,\ (\tau\mbox{-}\tau_1) \text{ sürekli} )(\pi_2:X\times Y\to Y,\,\ (\tau\mbox{-}\tau_2) \text{ sürekli} )
\Rightarrow
(A_1\in \tau_1\Rightarrow \pi_1^{-1}[A_1]=A_1\times Y\in\tau)(A_2\in \tau_2\Rightarrow \pi_2^{-1}[A_2]=X\times A_2\in\tau)
\Rightarrow
[(A_1\in \tau_1)(A_2\in \tau_2)\Rightarrow \pi_1^{-1}[A_1]\cap \pi_2^{-1}[A_2]=(A_1\times Y)\cap (X\times A_2)=(A_1\cap X)\times (Y\cap A_2)=A_1\times A_2\in \tau]\ldots (4)
Öte yandan
A\in\tau_1\star\tau_2\Rightarrow (\exists\mathcal{A}_1\subseteq \tau_1)(\exists\mathcal{A}_2\subseteq \tau_2)(A=\cup_{(A_1\in\mathcal{A}_1)(A_2\in\mathcal{A}_2)}(A_1\times A_2))\ldots (5)
Buradan (4),(5)\Rightarrow A\in\tau\Big{/}\tau_1\star\tau_2\subseteq\tau\ldots (6) elde edilir. O halde
(3),(6)\Rightarrow \tau_1\star\tau_2=\min\mathcal{T}.