A031980 a(n) is the smallest number >= 1 not occurring earlier and not the sum of cubes of two distinct earlier terms.

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

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-François 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: A004728 A376573 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