Kaoru Morisugi "On the group $\mathrm{Ext}^{1,*}_{A^{BP}}(BP^* (S^0),BP^* (S^0))$ and the Hurewicz image of $BP/S^0$.," Journal of Mathematics of Kyoto University, J. Math. Kyoto Univ. 15(3), 647-669, (1975)