Is this the mathematical equation for the Vigenere Cipher Is this the mathematical equation for the Vigenere Cipher or something else and if something else what is it?

Where t is the letter of plain text and n is the position of t within the text and c is the ciphered character.

For 't' and 'c' taking a as 1 and z as 26.

((t + n) &gt; 25 -&gt; c = (t + n) - 25) ^ ((t + n) &lt;= 25 -&gt; c = (t + n))
NO, the question does not contain the mathematical equation for the Vigenere Cipher with plaintext t and ciphertext c in thet set $\{1\dots 26\}$ (with the letter a as 1 and the letter z as 26), and displacement (or key) n, for two reasons:


the best interpretation I can make of the expression given is by considering -&gt; to be the $\implies$ mathematical symbol, and ^ to mean logical XOR (or perhaps logical OR), that is one of the left thing or the right thing holds (possible meanings of the word one are equivalent in the context); but even with these assumptions, for n of 1 (meaning next character, circularly) the expression given maps the plaintext y (coded by t of 25) to the ciphertext a (coded by c of 1) instead of the desired z (coded 26);
the expression given does not parse (to me) as a mathematical equation.


With the original formalism, the Vigenere Cipher with plaintext t and ciphertext c in $\{1\dots 26\}$, and displacement n in the set $\{0\dots 26\}$, a correct expression would be:

((t + n) &gt; 26 -&gt; c = (t + n) - 26) ^ ((t + n) &lt;= 26 -&gt; c = (t + n))

and a passable mathematical equation would be:
$$t\mapsto c=((t+n+25)\bmod 26)+1$$

If we use the set $\{0\dots 25\}$ for plaintext and ciphertext, we get the nicer
$$t\mapsto c=(t+n)\bmod 26$$

or in the original formalism the expression

((t + n) &gt; 25 -&gt; c = (t + n) - 26) ^ ((t + n) &lt;= 25 -&gt; c = (t + n))