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praeclarum theorema
The praeclarum theorema, or splendid theorem, is a theorem of propositional calculus that was noted and named by G.W. Leibniz, who stated and proved it in the following manner:
If $a$ is $b$ and $d$ is $c$, then $ad$ will be $bc$.
This is a fine theorem, which is proved in this way:
$a$ is $b$, therefore $ad$ is $bd$ (by what precedes),
$d$ is $c$, therefore $bd$ is $bc$ (again by what precedes),
$ad$ is $bd$, and $bd$ is $bc$, therefore $ad$ is $bc$. Q.E.D.
(Leibniz, Logical Papers, p. 41).
Expressed in contemporary logical notation, the praeclarum theorema (PT) may be written as follows:
$((a\Rightarrow b)\land(d\Rightarrow c))\Rightarrow((a\land d)\Rightarrow(b% \land c))$ 
Representing propositions as logical graphs under the existential interpretation, the praeclarum theorema is expressed by means of the following formal equation:
(1) 
And here’s a neat proof of that nice theorem.
(2) 
1 References
2 Readings

Sowa, John F. (2002), “Peirce’s Rules of Inference”, Online.
3 Resources

Dau, Frithjof (2008), Computer Animated Proof of Leibniz’s Praeclarum Theorema.

Megill, Norman (2008), Praeclarum Theorema @ Metamath Proof Explorer.
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