[ abs 0 swap
  [ base share /mod
    rot + swap
    dup 0 = until ]
  drop ]                           is digitsum    ( n --> n   )

[ 0 swap
  [ dup base share > while
    dip 1+
    digitsum again ] ]             is digitalroot ( n --> n n )

[ dup digitalroot
  rot echo
  say " has additive persistance "
  swap echo
  say " and digital root of "
  echo
  say ";" cr ]                     is task        ( n -->     )

627615 task
39390 task
588225 task
393900588225 task