1. Department of Applied Mathematics and the Institute of Natural Sciences, Kyung Hee University 1 , 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Republic of Korea
2. Department of Mathematics, University of Seoul 2 , 163 Seoulsiripdaero, Dongdaemun-gu, Seoul 02504, Republic of Korea
3. Department of Mathematics, Kunsan National University 3 , 558, Daehak-ro, Gunsan-si 54150, Jeollabuk-do, Republic of Korea