Synonyms: Cyclobutenediylium,1,3-bis[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2,4-dihydroxy-,dihydroxide, bis(inner salt); Cyclobutenediylium,1,3-dihydroxy-2,4-bis[(1,3,3-trimethyl-2-indolinylidene)methyl]-, dihydroxide,bis(inner salt) (8CI); NK 2990