  We study the flavor-changing-neutral-current process c to u mu+ mu- using 1.3
fb^-1 of p p bar collisions at sqrt(s) = 1.96 TeV recorded by the D0 detector
operating at the Fermilab Tevatron Collider. We see clear indications of the
Ds+ and D+ to phi pi+ to mu+ mu- pi+ final states with significance greater
than four standard deviations above background for the D+ state. We search for
the continuum decay of D+ to pi+mu+mu- in the dimuon invariant mass spectrum
away from the phi resonance. We see no evidence of signal above background and
set a limit of B(D+ to pi+mu+mu-) < 3.9 x 10^-6 at the 90% C.L. This limit
places the most stringent constraint on new phenomena in the c to u mu+ mu-
transition.
