Lower bounds for independence and $k$-independence number of graphs using the concept of degenerate degrees

Let $G$ be a graph and $v$ any vertex of $G$. We define the degenerate degree of $v$, denoted by $ζ(v)$ as $ζ(v)={\max}_{H: v\in H}~δ(H)$, where the maximum is taken over all subgraphs of $G$ containing the vertex $v$. We show that the degenerate degree sequence of any graph can be determined by an efficient algorithm. A $k$-independent set in $G$ is any set $S$ of vertices such that $Δ(G[S])\leq k$. The largest cardinality of any $k$-independent set is denoted by $α_k(G)$. For $k\in \{1, 2, 3\}$, we prove that $α_{k-1}(G)\geq {\sum}_{v\in G} \min \{1, 1/(ζ(v)+(1/k))\}$. Using the concept of cheap vertices we strengthen our bound for the independence number. The resulting lower bounds improve greatly the famous Caro-Wei bound and also the best known bounds for $α_1(G)$ and $α_2(G)$ for some families of graphs. We show that the equality in our bound for independence number happens for a large class of graphs. Our bounds are achieved by Cheap-Greedy algorithms for $α_k(G)$ which are designed by the concept of cheap sets. At the end, a bound for $α_k(G)$ is presented, where $G$ is a forest and $k$ an arbitrary non-negative integer.

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