  Let $P_k$ be the graded polynomial algebra $\mathbb F_2[x_1,x_2,\ldots ,x_k]$
with the degree of each generator $x_i$ being 1, where $\mathbb F_2$ denote the
prime field of two elements, and let $GL_k$ be the general linear group over
$\mathbb F_2$ which acts regularly on $P_k$. We study the algebraic transfer
constructed by Singer using the technique of the Peterson hit problem. This
transfer is a homomorphism from the homology of the mod-2 Steenrod algebra
$\mathcal A$, $\text{Tor}^{\mathcal A}_{k,k+d} (\mathbb F_2,\mathbb F_2)$, to
the subspace of $\mathbb F_2{\otimes}_{\mathcal A}P_k$ consisting of all the
$GL_k$-invariant classes of degree $d$. In this paper, by using the results on
the Peterson hit problem we present the proof of the fact that the Singer
algebraic transfer is an isomorphism for $k \leqslant 3$. We also explicitly
determine the fourth Singer algebraic transfer in some degrees.
