Hwa-Long Gau is Professor in the Department of Mathematics at National Central University, Taiwan. Together with Pei Yuan Wu, he has co-authored over 40 publications on numerical range problems. One of them, Zero-dilation index of a finite matrix (2014), is currently the most-downloaded article in 'Linear Algebra and its Applications'. Pei Yuan Wu is Professor Emeritus in the Department of Applied Mathematics of National Chiao Tung University. He has been working in operator theory and matrix analysis for 45 years, recently focusing on the numerical ranges of operators and matrices. He was awarded the 16th Bela Szokefalvi-Nagy Medal by the Bolyai Institute of University of Szeged in 2015.