Centuries: 6th century - 7th century - 8th century
Decades: 590s 600s 610s 620s 630s - 640s - 650s 660s 670s 680s 690s
Years: 642 643 644 645 646 - 647 - 648 649 650 651 652
... to characterise as a continuous quantity.
Geographers and mathematicians have made a number of attempts to formalize the concept of population density.
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