An NSW number (named after Newman, Shanks, and Williams) is an integer 
that solves the Diophantine equation
 |
(1)
|
In other words, the NSW numbers 
index the diagonals of squares of side length 
having the property that the squares of the diagonal 
equals one plus a square
number 
.
Such numbers were called "rational diagonals" by the Greeks (Wells 1986,
p. 70). The name "NSW number" derives from the names of the authors
of the paper on the subject written by Newman et al. (1980/81).
The first few NSW numbers are therefore 
, 7, 41, 239, 1393, ... (OEIS A002315),
which correspond to square side lengths 
, 5, 29, 169, 985, 5741, 33461, 195025, ... (OEIS A001653).
The values indexed by 
and 
therefore give 2, 50, 1682, 57122, ...
(OEIS A088920).
Taking twice the NSW numbers gives the sequence 2, 14, 82, 478, 2786, 16238, ... (OEIS A077444), which is exactly every other Pell-Lucas number.
The first few prime NSW numbers are 
, 41, 239, 9369319, 63018038201, 489133282872437279, ...
(OEIS A088165), corresponding to indices 
, 2, 3, 9, 14, 23, 29, 81, 128, 210,
468, 473, 746, 950, 3344, 4043, 4839, 14376, 39521, 64563, 72984, 82899, 84338, 85206,
86121, 139160, ... (OEIS A113501).
The following table summarizes the largest known NSW primes, where the indices 
correspond via 
to the indices 
of prime half-Pell-Lucas numbers
that are odd.
 | decimal digits | discoverer | date |
 |  | E. W. Weisstein | May 19, 2006 |
 |  | E. W. Weisstein | Aug. 29, 2006 |
 |  | E. W. Weisstein | Nov. 16, 2006 |
 |  | E. W. Weisstein | Nov. 26, 2006 |
 |  | E. W. Weisstein | Dec. 10, 2006 |
 |  | E. W. Weisstein | Jan. 25, 2007 |
 |  | R. Price | Dec. 7, 2018 |
Interestingly, the values 
give every other convergent to Pythagoras's constant 
.
Explicit formula for 
and 
are given by
 |  |  |
(2)
|
 |  |  |
(3)
|
for positive integers 
(Ribenboim 1996, p. 367). A recurrence relation
for 
is given by
 |
(4)
|
with 
and 
.
