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Of the anions of structural formulae (A-1) to (A-43), those of formulae (A-1) to (A-23) are especially preferred for availability of starting carboxylic acids and ease of synthe- 55 SIS. 100 

In formula (a), R , R $^{200 }$and R$^{300 }$are each independently a straight C$_{1}$-C$_{20 }$or branched or cyclic C$_{3}$-C$_{20 }$monovalent hydrocarbon group which may be substituted with or sepa› rated by a heteroatom. Examples of the monovalent hydro- 60 carbon group are as exemplified for R 11 , R 22 , R 33 , R 44 , R 55 and R$^{01 }$in formula (1). Also, any two or more ofR 100 , R 200 and R$^{300 }$may bond together to form a ring with the sulfur atom in formula (a), and the ring may have a carbonyl moiety, sulfur atom, S(=O) moiety, S0$_{2 }$ moiety, oxygen 65 atom, or NH moiety interposed therein. In this case, exem› plary structures are shown below. 

-continued 

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Herein, R$^{600 }$is a monovalent hydrocarbon group as exem- plified above for R 100 , R $^{200 }$and R 300 . 

Preferred structures for the sulfonium cation having for- mula (a) are shown below although the invention is not limited thereto. 

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