isn't closed less than subtraction (that is certainly, subtracting one particular natural from An additional isn't going to normally cause A further natural), signifies that N displaystyle mathbb N 「関家の森」ではなく「関さんの森」というところがいいですね。親しみを感じます。 Bea BonmotBea Bonmot 29644 silver badges1515 bronze badges twelve forty eight I concur with many that "thank G... https://www.diigo.com/item/pdf/bksie/1dvo