

A252487


Smallest k such that n^7 = a_1^7 + ... + a_k^7 and all a_i are positive integers less than n.


1



128, 28, 66, 39, 28, 26, 21, 20, 18, 22, 22, 22, 20, 21, 14, 17, 14, 14, 17, 16, 17, 14, 16, 13, 15, 13, 12, 15, 13, 15, 13, 14, 13, 14, 13, 13, 14, 12, 12, 12, 13, 12, 12, 12, 11, 13, 13, 12, 12, 13, 12, 12, 11, 12, 11, 11, 12, 12, 11, 12, 9, 12, 11, 11, 11
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OFFSET

2,1


COMMENTS

Inspired by Fermat's Last Theorem: 2 never occurs in this sequence.
No n is known for which a(n)<7, according to the MathWorld page. The values 7, 8, 9, ... occur first at indices 568, 102, 62, ...
I conjecture that the sequence is bounded by the initial term a(2)=128. Probably even a(4)=66, a(5)=39, a(6)=28 and some more are followed only by smaller terms.
I've uploaded two scripts; one to compute the bfile and one to generate an IP file. For the first script, a parameter kmax can be set to gain a speedup but more memory is used. The other one (which also works with large integers now) should be used in case someone has a good IPsolver. Higher terms might be computable faster with a good IP solver.  Manfred Scheucher, Aug 14 2015
From results on Waring's problem, it is known that all a(n) <= A002804(7) = 143, and a(n) <= 33 for all sufficiently large n.  Robert Israel, Aug 16 2015


LINKS

Giovanni Resta, Table of n, a(n) for n = 2..200
JeanCharles Meyrignac, Computing Minimal Equal Sums Of Like Powers
Manfred Scheucher, Sage Script for IPgeneration
Manfred Scheucher, Sage Script for bfile generation
Eric Weisstein's World of Mathematics, Diophantine Equation7th Powers
Eric Weisstein's World of Mathematics, Waring's Problem


MAPLE

M:= 10^8:
R:= Vector(M, 144, datatype=integer[4]):
for p from 1 to floor(M^(1/7)) do
p7:= p^7;
if p > 1 then A[p]:= R[p7] fi;
R[p7]:= 1;
for j from p7+1 to M do
R[j]:= min(R[j], 1+R[j  p7]);
od
od:
F:= proc(n, k, ub)
local lb, m, bestyet, res;
if ub <= 0 then return 1 fi;
if n <= M then
if n = 0 then return 0
elif R[n] > ub then return 1
else return R[n]
fi
fi;
lb:= floor(n/k^7);
if lb > ub then return 1 fi;
bestyet:= ub;
for m from lb to 0 by 1 do
res:= procname(nm*k^7, k1, bestyetm);
if res >= 0 then
bestyet:= res+m;
fi
od:
return bestyet
end proc:
for n from floor(M^(1/7))+1 to 50 do
A[n]:= F(n^7, n1, 144)
od:
seq(A[n], n=2..50); # Robert Israel, Aug 17 2015


PROG

(PARI) a(n, verbose=0, m=7)={N=n^m; for(k=3, 999, forvec(v=vector(k1, i, [1, n\sqrtn(k+1i, m)]), ispower(Nsum(i=1, k1, v[i]^m), m, &K)&&K>0&&!(verbose&&print1("/*"n" "v"*/"))&&return(k), 1))}


CROSSREFS

Cf. A002804, A161882, A161883, A161884, A161885, A252486.
Sequence in context: A035880 A010369 A121374 * A160638 A188829 A172532
Adjacent sequences: A252484 A252485 A252486 * A252488 A252489 A252490


KEYWORD

nonn


AUTHOR

M. F. Hasler, Dec 17 2014


EXTENSIONS

More terms from Manfred Scheucher, Aug 15 2015
a(50)a(66) from Giovanni Resta, Aug 17 2015


STATUS

approved



