A306213 Numbers that are the sum of cubes of three distinct positive integers in arithmetic progression.

36, 99, 153, 216, 288, 405, 408, 495, 645, 684, 792, 855, 972, 1071, 1197, 1224, 1407, 1548, 1584, 1701, 1728, 1968, 2079, 2241, 2304, 2403, 2541, 2673, 2736, 3051, 3060, 3240, 3264, 3537, 3540, 3888, 3960, 4059, 4131, 4257, 4500, 4587, 4833, 5049, 5160, 5256, 5472, 5643, 5832, 5940, 6336, 6369, 6669

Offset

1,1

Comments

Numbers that can be written as 3*a*(a^2 + 2*b^2) = (a-b)^3 + a^3 + (a+b)^3 where 0 < b < a. - Robert Israel, Dec 15 2022

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

153 = 1^3 + 3^3 + 5^3, with 3 - 1 = 5 - 3 = 2;
972 = 3^3 + 6^3 + 9^3, with 6 - 3 = 9 - 6 = 3.

Maple

N:= 10000: # for terms <= N
S:= {}:
for a from 1 while a^3 + (a+1)^3 + (a+2)^3 <= N do
  for d from 1 do
    x:= a^3 + (a+d)^3 + (a+2*d)^3;
    if x > N then break fi;
    S:= S union {x}
od od:
sort(convert(S, list)); # Robert Israel, Dec 14 2022

Prog

(PARI) for(n=3, 7000, k=(n/3)^(1/3); a=2; v=0; while(a<=k&&v==0, b=(n-3*a^3)/(6*a); if(b==truncate(b)&&issquare(b), d=sqrt(b), d=0); if(d>=1&&d<=a-1, v=1; print1(n, ", ")); a+=1))
(PARI) w=List(); for(n=3, 7000, k=(n/3)^(1/3); for(a=2, k, for(c=1, a-1, v=(a-c)^3+a^3+(a+c)^3; if(v==n, listput(w, n))))); print(vecsort(Vec(w), , 8))

Crossrefs

Cf. A000578, A023042, A024975, A122723, A306212, A306214.

Sequence in context: A287613 A118632 A260007 * A114819 A137945 A250812

Adjacent sequences: A306210 A306211 A306212 * A306214 A306215 A306216

Keyword

nonn

Author

Antonio Roldán, Jan 29 2019

Status

approved