login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A269998
Denominators of r-Egyptian fraction expansion for 1/Pi, where r = (1,1/2,1/3,1/4,...)
2
4, 8, 58, 3984, 22875462, 931267108879599, 1031674577884217945682977326053, 1260295551033259417770370489346530643885445465368122822066849
OFFSET
1,1
COMMENTS
Suppose that r is a sequence of rational numbers r(k) <= 1 for k >= 1, and that x is an irrational number in (0,1). Let f(0) = x, n(k) = floor(r(k)/f(k-1)), and f(k) = f(k-1) - r(k)/n(k). Then x = r(1)/n(1) + r(2)/n(2) + r(3)/n(3) + ... , the r-Egyptian fraction for x.
See A269993 for a guide to related sequences.
EXAMPLE
1/Pi = 1/4 + 1/(2*8) + 1/(3*58) + ...
MATHEMATICA
r[k_] := 1/k; f[x_, 0] = x; z = 10;
n[x_, k_] := n[x, k] = Ceiling[r[k]/f[x, k - 1]]
f[x_, k_] := f[x, k] = f[x, k - 1] - r[k]/n[x, k]
x = 1/Pi; Table[n[x, k], {k, 1, z}]
PROG
(PARI) r(k) = 1/k;
x = 1/Pi;
f(x, k) = if(k<1, x, f(x, k - 1) - r(k)/n(x, k));
n(x, k) = ceil(r(k)/f(x, k - 1));
for(k = 1, 8, print1(n(x, k), ", ")) \\ Indranil Ghosh, Mar 29 2017
CROSSREFS
Cf. A269993.
Sequence in context: A063083 A349859 A270399 * A335527 A303284 A275574
KEYWORD
nonn,frac,easy
AUTHOR
Clark Kimberling, Mar 15 2016
STATUS
approved