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The Lagrangian density of an $r$-uniform hypergraph $H$ is $r!$ multiplying the supremum of the Lagrangians of all $H$-free $r$-uniform hypergraphs. For an $r$-uniform graph $H$ with $t$ vertices, it is clear that $π_λ(H)\ge r!λ{(K_{t-1}^r)}$. We say that an $r$-uniform hypergraph $H$ with $t$ vertices is $λ$-perfect if $π_λ(H)= r!λ{(K_{t-1}^r)}$. A theorem of Motzkin and Straus implies that all $2$-uniform graphs are $λ$-perfect. It is interesting to understand what kind of hypergraphs are $λ$-perfect. The property `$λ$-perfect' is monotone in the sense that an $r$-graph obtained by removing an edge from a $λ$-perfect $r$-graph (keep the same vertex set) is $λ$-perfect. It's interesting to understand the relation between the number of edges in a hypergraph and the `$λ$-perfect' property. We propose that the number of edges in a hypergraph no more than the number of edges in a linear hyperpath would guarantee the `$λ$-perfect' property. We show some partial result to support this conjecture. We also give some partial result to support the conjecture that the disjoint union of two $λ$-perfect $r$-uniform hypergraph is $λ$-perfect. We show that the disjoint union of a $λ$-perfect $3$-graph and $S_{2,t}=\{123,124,125,126,...,12(t+2)\}$ is perfect. This result implies the earlier result of Heftz and Keevash, Jiang, Peng and Wu, and several other earlier results.