It's _inside_ the bos.adt fileset, which is on the first CD. Execute smit installp, input the correct directory,and manually select the fileset to install be pressing F4. Scroll down until you find the bos.adt stuff. Select libm with F7.[@more@]

Oracle 10g pre-required checking can use:

lslpp -l bos.adt.base bos.adt.lib bos.adt.libm bos.perf.perfstat bos.perf.libperfstat bos.perf.proctools rsct.basic.rte xlC.rte xlC.aix50.rte