Triarylphosphine or triarylarsine compound, alpha-olefin polymerization catalyst using the compound, ternary copolymer, and production process of alpha-olefin-((meth)acrylic acid)-based copolymer

A ternary copolymer of an ethylene, an α-olefin having a carbon number of 3 to 10 and a (meth)acrylic acid or ester represented by CH2═C(R18)CO2(R19), wherein R18 is a hydrogen atom or an alkyl group having a carbon number of 1 to 10, and R19 is a hydrogen atom or an alkyl group having a carbon number of 1 to 30, which may contain a hydroxyl group, an alkoxy group or an epoxy group on an arbitrary position, the ternary copolymer satisfying the following requirements (a) and (b):

• (a) the ratio Mw/Mn of the weight average molecular weight (Mw) to the number average molecular weight (Mn) satisfies the following relationship:
• 1.5≦Mw/Mn≦3
• (b) the melting point Tm (° C.), the α-olefin content [C] (mol %) and the polar group-containing vinyl monomer content [X] (mol %) from the (meth)acrylic acid or ester satisfy the following relationship:
• 60≦Tm≦135−6.4×([C]+[X])
• wherein Tm is a peak temperature of a melting curve obtained by the measurement using a differential scanning calorimeter (DSC), and when a plurality of melting peaks are detected, Tm is the temperature of the maximum peak out of detected peaks.