an A4 print on a pin board, having a flow chart saying: Are you single? Do you love math? Prove it! And a 4 times 4 matrix containing various complicated mathematical expressions. The determinant of the matrix is her phone number, which you are allowed to call if you are approximately her age (27 to 35). I censored a part to hopefully obscure her number enough.
https://yuustan.space/files/webpublic-b01b6dc3-e834-4b21-ac71-1becf2806cf2