Loeschian number

Loeschian number
Named after	August Lösch
Subsequence of	A032766
Formula	x2 + xy + y2 for integer x, y
First terms	0, 1, 3, 4, 7, 9, 12, 13, 16
OEIS index	A003136; Loeschian numbers;

In number theory, the numbers of the form x² + xy + y² for integer x, y are called the Loeschian numbers. These numbers are named after August Lösch. They are the norms of the Eisenstein integers. They are a set of whole numbers, including zero, and having prime factorization in which all primes congruent to 2 mod 3 have even powers (there is no restriction of primes congruent to 0 or 1 mod 3).

References

Marshall, J. U. (1975). "The Loeschian numbers as a problem in number theory". Geographical Analysis. 7 (4): 421–426. doi:10.1111/j.1538-4632.1975.tb01054.x.
"A003136". On-Line Encyclopedia of Integer Sequences. Retrieved 19 July 2018.

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