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A024670
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Numbers that are sums of 2 distinct positive cubes.
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42
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9, 28, 35, 65, 72, 91, 126, 133, 152, 189, 217, 224, 243, 280, 341, 344, 351, 370, 407, 468, 513, 520, 539, 559, 576, 637, 728, 730, 737, 756, 793, 854, 855, 945, 1001, 1008, 1027, 1064, 1072, 1125, 1216, 1241, 1332, 1339, 1343, 1358, 1395, 1456, 1512, 1547, 1674
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OFFSET
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1,1
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COMMENTS
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This sequence contains no primes since x^3+y^3=(x^2-x*y+y^2)*(x+y). - M. F. Hasler, Apr 12 2008
There are no terms == 3, 4, 5 or 6 mod 9. - Robert Israel, Oct 07 2014
a(n) mod 2: {1,0,1,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,1,0,1,1,0,1,0,0,1,0,1,0,1,1,1,0, ...} - Daniel Forgues, Sep 27 2018
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LINKS
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EXAMPLE
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9 is in the sequence since 2^3 + 1^3 = 9.
35 is in the sequence since 3^3 + 2^3 = 35.
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MAPLE
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N:= 10000: # to get all terms <= N
S:= select(`<=`, {seq(seq(i^3 + j^3, j = 1 .. i-1), i = 2 .. floor(N^(1/3)))}, N);
# if using Maple 11 or earlier, uncomment the next line
# sort(convert(S, list));
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MATHEMATICA
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lst={}; Do[Do[x=a^3; Do[y=b^3; If[x+y==n, AppendTo[lst, n]], {b, Floor[(n-x)^(1/3)], a+1, -1}], {a, Floor[n^(1/3)], 1, -1}], {n, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jan 22 2009 *)
Select[Range@ 1700, Total@ Boole@ Map[And[! MemberQ[#, 0], UnsameQ @@ #] &, PowersRepresentations[#, 2, 3]] > 0 &] (* Michael De Vlieger, May 13 2017 *)
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PROG
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(PARI) isA024670(n)=for( i=ceil(sqrtn( n\2+1, 3)), sqrtn(n-.5, 3), isA000578(n-i^3) & return(1)) /* One could also use "for( i=2, sqrtn( n\2-1, 3), ...)" but this is much slower since there are less cubes in [n/2, n] than in [1, n/2]. Replacing the -1 here by +.5 would yield A003325, allowing for a(n)=x^3+x^3. Replacing -1 by 0 may miss some a(n) of this form due to rounding errors. - M. F. Hasler, Apr 12 2008 */
(Python)
from itertools import count, takewhile
def aupto(limit):
cbs = list(takewhile(lambda x: x <= limit, (i**3 for i in count(1))))
sms = set(c+d for i, c in enumerate(cbs) for d in cbs[i+1:])
return sorted(s for s in sms if s <= limit)
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CROSSREFS
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See also: Sums of 2 positive cubes (not necessarily distinct): A003325. Sums of 3 distinct positive cubes: A024975. Sums of distinct positive cubes: A003997. Sums of 2 distinct nonnegative cubes: A114090. Sums of 2 nonnegative cubes: A004999. Sums of 2 distinct positive squares: A004431. Cubes: A000578.
Cf. A373971 (characteristic function).
Indices of nonzero terms in A025468.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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