It is: 671 = DCLXXI in Roman numerals
10 + 671 = 681
671,845 is the number halfway between 671,840 and 671,850.
prime
625, the square of 25, is the only square number between 602 and 671.
currently number 671.
671
671
671
8.0843
671, 1342, 2013, 2684, 3355, 4026 +671 . . .
671 128 527