An application of the hit problem to the algebraic transfer
Let $P_k$ be the polynomial algebra $\mathbb F_2[x_1,x_2,\ldots ,x_k]$ over the field $\mathbb F_2$ with two elements, in $k$ variables $x_1, x_2, \ldots , x_k$, each variable of degree 1. Denote by $GL_k$ the general linear group over $\mathbb F_2$ which regularly acts on $P_k$. The algebra $P_k$ is a module over the mod-2 Steenrod algebra $\mathcal A$. In 1989, Singer [22] defined the $k$-th homological algebraic transfer, which is a homomorphism $$\varphi_k=(\varphi_k)_m :{\rm Tor}^{\mathcal A}_{k,k+m} (\mathbb F_2,\mathbb F_2) \to (\mathbb F_2\otimes_{\mathcal A}P_k)_m^{GL_k}$$ from the homological group of the mod-2 Steenrod algebra $\mbox{Tor}^{\mathcal A}_{k,k+m} (\mathbb F_2,\mathbb F_2)$ to the subspace $(\mathbb F_2\otimes_{\mathcal A}P_k)_m^{GL_k}$ of $\mathbb F_2{\otimes}_{\mathcal A}P_k$ consisting of all the $GL_k$-invariant classes of degree $m$. In general, the transfer $\varphi_k$ is not a monomorphism and Singer made a conjecture that $\varphi_k$ is an epimorphism for any $k \geqslant 0$. The conjecture is studied by many authors. It is true for $k \leqslant 3$ but unknown for $k \geqslant 4$. In this paper, by using the results of the Peterson hit problem for the polynomial algebra in four variables, we prove that Singer's conjecture for the fourth algebraic transfer is true in the families of generic degrees $d_{s,t} = 2^{s+t}+2^s-3$ and $n_{s,t}=2^{s+t}+2^s-2$ with $s,\, t$ positive integers. Our results also show that many of the results in Ph\'uc [16,17,18] are seriously false. The proofs of the results in Ph\'uc's works are only provided for a few special cases but they are false and incomplete.