OFFSET
1,1
LINKS
FORMULA
{n: A025428(n) >= 2}. - R. J. Mathar, Jun 15 2018
MAPLE
N:= 1000: # to get all terms <= N
V:= Vector(N):
for x from 1 while x^2 +3 <= N do
for y from 1 to x while x^2 + y^2 + 2 <= N do
for z from 1 to y while x^2 + y^2 + z^2 + 1 <= N do
for w from 1 to z while x^2 + y^2 + z^2 + w^2 <= N do
t:= x^2 + y^2 + z^2 + w^2;
V[t]:= V[t]+1;
od od od od:
select(t -> V[t] >= 2, [$1..N]); # Robert Israel, Jul 05 2017
MATHEMATICA
Select[Range@ 200, 2 == Length@ Quiet@ IntegerPartitions[#, {4}, Range[Sqrt@ #]^2, 2] &] (* Giovanni Resta, Jul 05 2017 *)
M = 1000;
Clear[V]; V[_] = 0;
For[a = 1, a <= Floor[Sqrt[M/4]], a++,
For[b = a, b <= Floor[Sqrt[(M - a^2)/3]], b++,
For[c = b, c <= Floor[Sqrt[(M - a^2 - b^2)/2]], c++,
For[d = c, d <= Floor[Sqrt[M - a^2 - b^2 - c^2]], d++,
m = a^2 + b^2 + c^2 + d^2;
V[m] = V[m] + 1;
]]]];
Select[Range[M], V[#] >= 2&] (* Jean-François Alcover, Mar 22 2019, after Robert Israel *)
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved