"Directional recurrence for infinite measure preserving Zd actions" by A. A. Sahin and Aimee S. A. Johnson, Professor of Mathematics and Statistics. For full text access, see this citation in Works, Swarthmore College's Institutional Repository: http://works.swarthmore.edu/fac-math-stat/162/
Next Generation Science Standards << Stop Common Core in Michigan - We were told the standards were only for English and Math. Not true, there will be standards for all subjects before they are through.