Is an article required before a math variable?

Is the definite article required before a variable used to abbreviate a physical quantity?

The f_t is the same for both cases at various values of the L_c. As the L_c increases, the f_r also shifts.

f_t is the same for both cases at various values of L_c. As L_c increases, f_r also shifts.

If the second sentence is okay, I am still a bit weirded out by starting a sentence with a math notation.

(L_c = Length of the channel, f_t = frequency of the transistor)

This is answered by recognising that L_c and f_t (or f_r) are names, referring to the quantities "channel length" and "transistor frequency" (and whatever the other one is).

You could call "channel length" Cuthbert. A sentence referring to Cuthbert wouldn't use an article:

Fred is the same for both cases at various values of Cuthbert. As Cuthbert increases, Fred also shifts.

Don't use an article for the names/symbols for the quantities.

There's no weirdness in starting a sentence with a name, however it's spelled. If it doesn't have a capital letter, don't use one. It's unusual, because names usually have a capital letter and sentences should start with one. But changing f_t into F_t could substantially change what you are referring to. That is far more important than stylistic custom concerning capital letters. However, many don't like this flouting of convention, in which case, it's necessary to use "The value of f_t ..."

See also the ELU question Is it okay to start a sentence with a Greek letter (variable)?

If f_t, f_r and L_c have been defined previously, the second sentence is clear, unambiguous, conventional and correct.

In mathematical and physical discourse, a function of a variable may be represented as a symbol. For example, we have your f_t , or f(t) as it might often be represented. The value f of the function depends on the value of the determining variable t. The symbol f_t represents the manner in which the value f of the function depends on the determining variable t. Once defined the symbol is understood as the representation of the functional relationship between the two. It is therefore redundant to use "The" with such defined symbols, whether or not they occur at the start of the sentence.

The alphanumeric expression of the function is irrelevant, as is the number of determining variables. This argument applies to any function. For example F(t), f_t, p_b, Φ_x, Gamma(x,y,z),Phi_μ.

It is correct usage to refer to the function f(t) in examples such as the following. In each case the function is being defined on its first use.

"Consider the function f(t)=t*t, where f has a quadratic dependence on t."

"The function sin(t) has a regular periodicity in t"