Mourning with a Father Who Is Sitting Shiva

Shalom u’vrachah. Do sons whose father is sitting shiva for his mother need to observe mourning with him only when he is in the same city?
A. If the father sits shiva in the morning in another city, and from the afternoon in our city — during the time that he is not in our city, must we observe mourning?
B. If I leave the city where the father is sitting shiva, must I continue to observe mourning?

Shalom u’vrachah 

A. Only when one is in the same city where his father or mother is sitting shiva does one observe, according to Ashkenazic custom, some aspects of mourning.

B. Only when you are with him in the same city. 

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