\[1,\ 4,\ 7,\ 10\ldots\]
\[a_{1} = 1;\ \ a_{2} = 4:\]
\[d = 7 - 4 = 3.\]
\[a_{n} = 1 + 3 \cdot (n - 1) =\]
\[= 3n - 3 + 1 = 3n - 2.\]
\[a_{n} = 3n - 2.\]