A002243 - OEIS

A002243

Numbers that are not the sum of 3 distinct triangular numbers.

1, 2, 3, 5, 6, 8, 12, 15, 20, 23, 33, 78

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..12.

MATHEMATICA

Complement[Range[100], Union[Total/@Subsets[Accumulate[Range[0, 12]], {3}]]] (* Harvey P. Dale, Sep 02 2014 *)

PROG

(PARI) is(n)=for(c=ceil((sqrt(24*n-15)+3)/6), (sqrt(8*n-7)-1)/2, my(t=n-c*(c+1)/2); for(b=sqrtint(t-1)+1, min((sqrt(8*n+1)-1)/2, c-1), if(ispolygonal(t-b*(b+1)/2, 3), return(0)))); 1

select(is, [1..100]) \\ Charles R Greathouse IV, Nov 25 2014

CROSSREFS

Cf. A000217, A002244.

Sequence in context: A002384 A096176 A330748 * A094763 A338916 A125559

Adjacent sequences: A002240 A002241 A002242 * A002244 A002245 A002246

KEYWORD

fini,full,nonn

AUTHOR

N. J. A. Sloane, Dan Hoey

STATUS

approved