A232090 Minimal possible denominator for a sum of the form 1 +/- 1/2 +/- 1/3 +/- ... +/- 1/n.

1, 2, 6, 12, 60, 20, 140, 280, 2520, 2520, 27720, 27720, 360360, 360360, 72072, 144144, 2450448, 272272, 5173168, 5173168, 739024, 739024, 16997552, 16997552, 424938800, 424938800, 11473347600, 11473347600, 332727080400, 332727080400

Offset: 1

Author

M. F. Hasler, Nov 18 2013

Keywords

• nonn

Comments

Differs from A203811 from a(18)=272272 on, and from A002805 and A231693 from a(15)=72072 on.

Links

• Giovanni Resta, Table of n, a(n) for n = 1..37
• F. T. Adams-Watters, Re: Reciprocal Recaman, SeqFan list, Nov 17 2013

Crossrefs

Cf. A061195 (minimal possible positive numerator).

Programs

Mathematica

nMax = 19; d = {0}; Table[d = Flatten[{d + 1/n, d - 1/n}]; Min[Denominator[d]], {n, nMax}] (* T. D. Noe, Nov 19 2013 *)

PARI

for(n=0,19,m=(n+1)!;for(k=0,2^n-1,m=min(denominator(sum(j=2,n+1,(-1)^bittest(k,j-2)/j,1)),m));print1(m","))

Extensions

Terms a(21)-a(30) from David W. Wilson, Nov 19 2013