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in Polyhedral Study of the Cardinality Constrained Knapsack Problem the authors prove that the Cardinality Constrained Knapsack Problem is NP-Hard by reducing PARTITION to it.

Besides, it's easy to see that the KP problem is a special case of the QKP.

How should one proceed to prove the NP-Hardness of the Cardinality Constrained Quadratic Knapsack Problem?

CCQKP:

$$max\ \sum_{i=1}^{n} \sum_{j=1}^{n} x_i x_j c_{ij}$$ $s.t$ $$\sum_{j=1}^{n} a_j x_j \leq C$$ $$\sum_{j=1}^{n}x_j = 1$$ $$0 \leq x_j \leq 1,\ j = 1,...,n$$ $$\sum_{j=1}^{n}z_j = k$$ $$z_j \in \{0,1\},\ j = 1,...,n$$

I know we usually talk about the hardness of Decision Problems even tough I'm formalizing the Optimization version of it.

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