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Yes, it is always possible to find regular triangle-free graphs of any degree up to half the number of vertices (as long as the number of vertices is even). To see this, first consider $K_{n,n}$. By Hall's Theorem, $K_{n,n}$ has a perfect matching $M$. Removing the edges of $M$ leaves a $(n-1)$-regular graph which is bipartite (and hence triangle-free). Repeat.

Note that

It is obviously not true if the number of vertices is odd. If $n$ and $k$ are both odd, then there are no $k$-regular graphs on $n$ vertices and hence no triangle-free ones either.

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Yes, it is always possible to find regular triangle-free graphs of any degree up to half the number of vertices. To see this, first consider $K_{n,n}$. By Hall's Theorem, $K_{n,n}$ has a perfect matching $M$. Removing the edges of $M$ leaves a $(n-1)$-regular graph which is bipartite (and hence triangle-free). Repeat.

Note that if $n$ and $k$ are both odd, then there are no $k$-regular graphs on $n$ vertices and hence no triangle-free ones either.

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Yes, it is always possible to find regular triangle-free graphs of any degree up to half the number of vertices. To see this, first consider $K_{n,n}$. By Hall's Theorem, $K_{n,n}$ has a perfect matching $M$. Removing the edges of $M$ leaves a $(n-1)$-regular graph which is bipartite (and hence triangle-free). Repeat.