![[Photo: Factor Stencil]](42_factor_stencil.jpg)
From the interpretive text: "John D. Elder, University of Michigan, 1939
The American number theorist Derrick Norman Lehmer designed a set of punched stencils that could be used to factor large numbers using the method of quadratic residues. Elder streamlined this procedure using a set of punched cards. By selecting appropriate cards and placing them atop one another, he could factor numbers larger than one billion."
![[^]](../../../images/end.gif){kind=small}