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A031980 a(n) = smallest number >= 1 not occurring earlier and not the sum of cubes of two distinct earlier terms. 6
1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32, 33, 34, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

Mihaly Bencze [Beneze], Smarandache recurrence type sequences, Bulletin of pure and applied sciences, Vol. 16E, No. 2, 1997, pp. 231-236.

F. Smarandache, Properties of numbers, ASU Special Collections, 1973.

LINKS

Klaus Brockhaus, Table of n, a(n) for n = 1..4900

F. Smarandache, Sequences of Numbers Involved in Unsolved Problems.

Eric Weisstein's World of Mathematics, Smarandache Sequences

MATHEMATICA

A031980 = {1}; Do[ m = Ceiling[(n-1)^(1/3)]; s = Select[ A031980, # <= m &]; ls = Length[s]; sumOfCubes = Union[ Flatten[ Table[ s[[i]]^3 + s[[j]]^3, {i, 1, ls}, {j, i+1, ls}]]]; If[ FreeQ[ sumOfCubes, n], AppendTo[ A031980, n] ], {n, 2, 77}]; A031980 (* Jean-Fran├žois Alcover, Dec 14 2011 *)

PROG

(MAGMA) m:=77; a:=[]; a2:={}; for n in [1..m] do p:=1; u:= a2 join { x: x in a }; while p in u do p:=p+1; end while; if p gt m then break; end if; a2:=a2 join { x^3 + p^3: x in a | x^3 + p^3 le m }; Append(~a, p); end for; print a; /* Klaus Brockhaus, Jul 16 2008 */

CROSSREFS

Cf. A024670 (sums of cubes of two distinct positive integers), A001235 (sums of two cubes in more than one way), A141805 (complement).

Sequence in context: A265556 A004728 A072886 * A183221 A317492 A324721

Adjacent sequences:  A031977 A031978 A031979 * A031981 A031982 A031983

KEYWORD

nonn,nice,easy

AUTHOR

J. Castillo (arp(AT)cia-g.com) [Broken email address?]

EXTENSIONS

More terms from Larry Reeves (larryr(AT)acm.org), Sep 26 2000

Better definition from Klaus Brockhaus, Jul 16 2008

STATUS

approved

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Last modified October 21 14:49 EDT 2019. Contains 328301 sequences. (Running on oeis4.)