Euler–Lagrange equation – 4-14-12
http://en.wikipedia.org/wiki/Euler%E2%80%93Lagrange_equation
http://mathworld.wolfram.com/Euler-LagrangeDifferentialEquation.html
http://mathsci.kaist.ac.kr/~nipl/am621/lecturenotes/Euler-Lagrange_equation.pdf
http://arxiv.org/abs/1203.2736
http://www.youtube.com/watch?v=Xr6-gZzgtFc
http://www.youtube.com/watch?v=E7FV9aaiwKQ&feature=related
http://www.youtube.com/watch?v=WT_IVdCO4ZE&feature=relmfu
http://www.youtube.com/watch?v=g7bakDI_TwA&feature=relmfu
http://www.youtube.com/watch?v=lWGb-pIAQ-E&feature=relmfu
