A091291 Numbers k such that the quotient (sigma(k) + sigma(k+1))/sigma(2*k+1) is smaller than 1/2.

1283782, 3445942, 4662157, 5743237, 7770262, 8806297, 8986477, 9572062, 9920137, 10472962, 11356537, 11554042, 12635122, 12870742, 14932417, 14977462, 15833317, 15878362, 17995477, 18761242, 19256737, 20202682, 21058537, 21824302, 22860337, 23709262, 24121597

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..1000

Mathematica

f[x_] := (DivisorSigma[1, x]+DivisorSigma[1, x+1])/DivisorSigma[1, 2*x+1];
Do[s=f[n]; If[ s < 1/2, Print[n]], {n, 10^7}]

Prog

(PARI) list(lim) = {my(s1 = 1, s2); for(k = 2, lim, s2 = sigma(k); if(s1+s2 < sigma(2*k-1)/2, print1(k-1, ", ")); s1 = s2); } \\ Amiram Eldar, Feb 11 2025

Crossrefs

Cf. A000203 (sigma), A067171, A091287, A091289, A091290.

Sequence in context: A328256 A118419 A250619 * A205178 A205663 A205952

Adjacent sequences: A091288 A091289 A091290 * A091292 A091293 A091294

Keyword

nonn

Author

Labos Elemer, Feb 16 2004

Extensions

a(15)-a(27) from Amiram Eldar, Feb 11 2025

Status

approved