Mingchuan ZhangYanmin YinZhongli WeiPositive solution of singular higher-order m-point boundary value problem with nonlinearity that changes sign.678-6872008201Appl. Math. Comput.1-2https://doi.org/10.1016/j.amc.2008.01.004db/journals/amc/amc201.html#ZhangYW08