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Omni-MATH / At a conference there are n mathematicians. Each of them knows exactly k fellow mathematicians. Find the smallest value of k such that there are at least three mathematicians that …

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problem

At a conference there are nn mathematicians. Each of them knows exactly kk fellow mathematicians. Find the smallest value of kk such that there are at least three mathematicians that are acquainted each with the other two. [color=#BF0000]Rewording of the last line for clarification:[/color] Find the smallest value of kk such that there (always) exists 33 mathematicians X,Y,ZX,Y,Z such that XX and YY know each other, XX and ZZ know each other and YY and ZZ know each other.
Plain-text mathematical notation (without MathML)
At a conference there are n mathematicians. Each of them knows exactly k fellow mathematicians. Find the smallest value of k such that there are at least three mathematicians that are acquainted each with the other two.

[color=#BF0000]Rewording of the last line for clarification:[/color]
Find the smallest value of k such that there (always) exists 3 mathematicians X,Y,Z such that X and Y know each other, X and Z know each other and Y and Z know each other.
Original LaTeX notation
At a conference there are $n$ mathematicians. Each of them knows exactly $k$ fellow mathematicians. Find the smallest value of $k$ such that there are at least three mathematicians that are acquainted each with the other two.

[color=#BF0000]Rewording of the last line for clarification:[/color]
Find the smallest value of $k$ such that there (always) exists $3$ mathematicians $X,Y,Z$ such that $X$ and $Y$ know each other, $X$ and $Z$ know each other and $Y$ and $Z$ know each other.

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