Number Sequence Analysis: Is -1 in the Series 51, 47, 43, 39, ...?

We are asked to determine whether the number $-1$ is part of the sequence $51, 47, 43, 39, \ldots$.

This is an arithmetic sequence where the first term $a_1 = 51$ and the common difference $d = -4$.

The formula for the $n$-th term of an arithmetic sequence is given by: $a_n = a_1 + (n-1) \cdot d$

Substituting the known values, we get: $a_n = 51 + (n-1)(-4)$

We want to find $n$ such that $a_n = -1$. Thus, $-1 = 51 + (n-1)(-4)$

Expanding and simplifying yields: $-1 = 51 - 4n + 4$
$-1 = 55 - 4n$
$4n = 55 + 1$
$4n = 56$
$n = \frac{56}{4}$
$n = 14$

Since $n = 14$ is a positive integer, $-1$ is indeed part of the sequence, appearing as the 14th term.

Therefore, the correct answer to the problem is Yes.