The symbol most commonly used to denote the adjoint operator. The dagger is also known as the obelisk, obelus, or long cross (Bringhurst 1997, p. 275).

Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, 1997.

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Weisstein, Eric W. "Dagger." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Dagger.html