in CURED, 1 min ago, by
Martin Sleziak @MartinR I will just add that A0 distinguishes between || and \| (I think) - so it might be useful to try both when searching for norms: https://approach0.xyz/search/?q=%24l%5E%5Cinfty%20%24%2C%20%24%5C%7Cx_n%2By_n%5C%7C_%7B%5Cinfty%7D%5Cleq%20%5C%7Cx_n%5C%7C_%7B%5Cinfty%7D%2B%5C%7Cy_n%5C%7C_%7B%5Cinfty%7D%24&p=1