A123204 Numbers that can be written as the sum of two squares in three ways, using three consecutive squares.

1, 25, 1105, 12025, 21025, 66625, 252601, 292825, 751825, 1026745, 1671865, 1892185, 4210945, 4534945, 8529625, 8958625, 10251025, 16040401, 24019801, 28404025, 29138425, 29604625, 47859265, 51396865, 53438905, 62747425

Offset

1,2

Comments

Numbers can be parameterized as follows: choose an odd number a and k a divisor of (a^2+1)/2, k <= sqrt((a^2+1)/2) (otherwise you will get a duplicate value with a negative number being squared) and let d = a/k * (a^2+3ak+2k^2+1) / k. Then the three consecutive squares are (d-1)/2, (d+1)/2 and (d+3)/2. The squares added to them are (d/a + a) / 2, (d/a - a) / 2 and ((d+2)/(a+2k) - (a+2k))/2. All members of this sequence are congruent to 1 or 25 (mod 120); all are the product exclusively of primes congruent to 1 (mod 4). No number can written as the sum of two squares using four consecutive squares.

Links

Table of n, a(n) for n=1..26.

Example

1105 = 31^2 + 12^2 = 32^2 + 9^2 = 33^2 + 4^2, using the consecutive values 31,32,33. 1 is included for the triple -1,0,1.

Crossrefs

Cf. A002144, A001481.

Sequence in context: A193121 A246923 A066852 * A012508 A112102 A012799

Adjacent sequences: A123201 A123202 A123203 * A123205 A123206 A123207

Keyword

nonn

Author

Franklin T. Adams-Watters, Oct 04 2006

Status

approved