Centered square number

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A centered square number is a centered figurate number that represents a square with a dot in the center and all other dots surrounding the center up to a certain city block distance, i.e. a diagonal square region of a square lattice, or an upright square region of a "centered square lattice". Like this:

The centered square number for n is given by the formula

[n^2 + (n + 1)^2.]

In other words, a centered square number is the sum of two consecutive square numbers. The following pattern demonstrates this formula:

The formula can also be expressed as

[ \over 2}]

but m has to be odd in this case.

The first few centered square numbers are

1, 5, 13, 25, 41, 61, 85, 113, 145, 181, 221, 265, 313, 365, 421, 481, 545, 613, 685, 761, 841, 925, 1013, 1105, 1201, 1301, 1405, 1513, 1625, 1741, 1861, 1985, 2113, 2245, 2381, 2521, 2665, 2813, 2965, 3121, 3281, 3445, 3613, 3785, 3961, 4141, 4325

All centered square numbers are odd, and in base 10 one can notice the one's digits follows the pattern 1-5-3-5-1.

All centered square numbers and their divisors have a remainder of one when divided by four. Hence all centered square numbers and their divisors end with digits 1 or 5 in base 6, 8 or 12.

Centered square prime

A centered square prime is a centered square number that is prime. The first few centered square primes are

5, 13, 41, 61, 113, 181, 313, 421, ....

External link

[(n^2 + 1) / 2 as a special case of M(i,j) = (i^2 + j) / 2]

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