A note on the double domination number in maximal outerplanar and planar graphs Noor A’lawiah Abd Aziz, Nader Jafari Rad and Hailiza Kamarulhaili RAIRO-Oper. Res., 56 5 (2022) 3367-3371 Published online: 14 September 2022 DOI: 10.1051/ro/2022150