How can we find the DOI of an old paper. For example, what are the DOI of the following papers?
Anderson, D.D. and Jayaram, C., 1995. Regular lattices. Studia Scientiarum Mathematicarum Hungarica, 30(3), pp.379-388.
Iorgulescu, A., 2004. Classes of BCK algebras-Part III. Preprint series of the Institute of Mathematics of the Romanian Academy, 3, pp.1-37.
Thakare, N.K., Manjarekar, C.S. and Maeda, S., 1988. Abstract spectral theory II: minimal characters and minimal spectrums of multiplicative lattices. Acta Sci. Math, 52, pp.53-67.