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Thanks! And in the summer, if it stands by an open window, is that okay? Otherwise, there is no way to take it outside ...

well, if not another way to keep it outside, then by an open window or just on another place with light. My mom takes them outside in spring and brings them inside in late autumn before the temperatures drop a lot. And her cacti are all fine and nice. My niece has some small cacti, but she keeps them always inside, during the whole year, and they are doing ok too. here they are.

You have good exemplars! Thanks for the advice, I will gradually learn, since I am a beginner cacti grower)