A006279 -id:A006279 - cp's OEIS frontend

A118227 Decimal expansion of Cahen's constant.

6, 4, 3, 4, 1, 0, 5, 4, 6, 2, 8, 8, 3, 3, 8, 0, 2, 6, 1, 8, 2, 2, 5, 4, 3, 0, 7, 7, 5, 7, 5, 6, 4, 7, 6, 3, 2, 8, 6, 5, 8, 7, 8, 6, 0, 2, 6, 8, 2, 3, 9, 5, 0, 5, 9, 8, 7, 0, 3, 0, 9, 2, 0, 3, 0, 7, 4, 9, 2, 7, 7, 6, 4, 6, 1, 8, 3, 2, 6, 1, 0, 8, 4, 8, 4, 4, 0, 8, 9, 5, 5, 5, 0, 4, 6, 3, 4, 3, 1, 9, 5, 4, 0, 5, 3

Offset: 0

Eric W. Weisstein, Apr 16 2006

Cahen proved that his constant is irrational. Davison and Shallit proved that it is transcendental and computed its simple continued fraction expansion A006280. - Jonathan Sondow, Aug 17 2014

Named after the French mathematician Eugène Cahen (1865 - 1941). - Amiram Eldar, Oct 29 2020

			0.6434105462883380261...

Steven R. Finch, Mathematical Constants, Encyclopedia of Mathematics and its Applications, vol. 94, Cambridge University Press, 2003, Section 6.7, p. 436.

Eugène Cahen, Note sur un développement des quantités numériques, qui présente quelque analogie avec celui en fractions continues, Nouvelles Annales de Mathématiques, Vol. 10 (1891), pp. 508-514.
J. L. Davison and Jeffrey Shallit, Continued Fractions for Some Alternating Series, Monatsh. Math., Vol. 111, No. 2 (1991), pp. 119-126, alternative link.
Eric Weisstein's World of Mathematics, Cahen's Constant.
Wikipedia, Cahen's constant.
Index entries for transcendental numbers.

Cf. A000058, A006279, A006280, A006281, A123180, A129871, A242724.

a[0] = 2; a[n_] := a[n] = a[n-1]^2 - a[n-1]+1; kmax = 1; FixedPoint[ RealDigits[ Sum[(-1)^k/(a[k]-1), {k, 0, kmax += 10}], 10, 105][[1]]&, kmax] (* Jean-François Alcover, Jul 28 2011, updated Jun 19 2014 *)
Most@First@RealDigits@N[x=1; 1+Sum[x=x(1+x); (-1)^k/x, {k, 1, 9}], 106] (* Oliver Seipel, Aug 25 2024, after Charles R Greathouse IV *)
Most@First@RealDigits@N[x=1; 1/2+Sum[x=x(1+x)(1+x+x^2); 1/(x+1), {k, 1, 4}], 106] (* Oliver Seipel, Aug 25 2024 *)

C=1;1+suminf(k=1,C+=C^2; (-1)^k/C) \\ Charles R Greathouse IV, Jul 14 2020

Equals Sum_{k >= 0} (-1)^k/(A000058(k)-1).
Equals Sum_{n>=0} 1/A000058(2*n) = 1 - Sum_{n>=0} 1/A000058(2*n+1). - Amiram Eldar, Oct 29 2020
Equals 1 + (1/2) * Sum_{n>=0} (-1)^(n+1)/A129871(n). - Bernard Schott, Apr 06 2021

A006280 Partial quotients in continued fraction expansion of Cahen's constant.

Original entry on oeis.org

0, 1, 1, 1, 4, 9, 196, 16641, 639988804, 177227652025317609, 72589906463585427805281295977816196, 2280022876287160141646375873338796324543839666085098409289740769448641

Offset: 0

N. J. A. Sloane

Shifted squares of denominators of convergents to Cahen's constant: a(n) = A006279(n-2)^2 for n > 1. - Jonathan Sondow, Aug 20 2014

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Amiram Eldar, Table of n, a(n) for n = 0..14
Khalil Ayadi, Chiheb Ben Bechir, and Maher Saadaoui, Continued Fractions with Predictable Patterns and Transcendental Numbers, J. Int. Seq. (2025) Vol. 28, Art. No. 25.1.4.
J. L. Davison and Jeffrey O. Shallit, Continued Fractions for Some Alternating Series, Monatshefte für Mathematik, Vol. 111 (1991), pp. 119-126; alternative link.

Cf. A006279, A006281, A118227.

b[n_] := b[n] = If[n < 2, 1, b[n-2]^2*b[n-1] + b[n-2]];
a[n_] := If[n == 0, 0, b[n-2]^2];
Table[a[n], {n, 0, 10}] (* Jean-François Alcover, Sep 23 2022, after Jonathan Sondow *)

A006281 Partial quotients in continued fraction expansion of 2C-1, where C is Cahen's constant.

Original entry on oeis.org

0, 3, 2, 18, 98, 33282, 319994402, 354455304050635218, 36294953231792713902640647988908098

Offset: 0

N. J. A. Sloane

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Amiram Eldar, Table of n, a(n) for n = 0..12
J. L. Davison and Jeffrey O. Shallit, Continued Fractions for Some Alternating Series, Monatshefte für Mathematik, Vol. 111 (1991), pp. 119-126; alternative link.

Cf. A006279, A006280, A118227.

s[0] = 2; s[n_] := s[n] = s[n - 1]^2 - s[n-1] + 1; h[0] = 1; h[n_] := h[n] = (s[n] - 1)/(2*h[n-1]); a[0] = 0; a[1] = 3; a[n_] := 2*h[n-1]^2; Array[a, 9, 0] (* Amiram Eldar, Mar 19 2024 *)

A123180 Even positions of Sylvester's sequence A000058; the denominators of the (greedy) Egyptian fraction expansion of Cahen's constant.

Original entry on oeis.org

2, 7, 1807, 10650056950807, 12864938683278671740537145998360961546653259485195807

Offset: 0

David Eppstein, Oct 03 2006

Amiram Eldar, Table of n, a(n) for n = 0..6
Eugène Cahen, Note sur un développement des quantités numériques, qui présente quelque analogie avec celui en fractions continues, Nouvelles Annales de Mathématiques, Vol. 10 (1891), pp. 508-514.
Eric Weisstein's World of Mathematics, Cahen's Constant.
Wikipedia, Cahen's constant.

Cf. A000058, A006279, A006280, A006281, A077125, A118227.

f[n_] := n*(n-1)*(n*(n-1)+1)+1; a[0] = 2; a[n_] := a[n] = f[a[n-1]]; Array[a, 5, 0] (* Amiram Eldar, Mar 19 2024 *)
1+NestList[#(#+1)(#^2+#+1) &, 1, 4] (* Oliver Seipel, Aug 25 2024 *)

a(n)=if(n, my(k=a(n-1));k*=k-1; k*(k+1)+1, 2) \\ Charles R Greathouse IV, Dec 12 2013

a(n) = a(n-1)*(a(n-1)-1)*(a(n-1)*(a(n-1)-1)+1)+1.
a(n) is approximately k^4^n with k = 1.5979102180318731783... (A077125). - Charles R Greathouse IV, Dec 12 2013
Sum_{n>=0} 1/a(n) = A118227. - Amiram Eldar, Mar 19 2024

a(4) from Charles R Greathouse IV, Dec 12 2013

A242724 Decimal expansion of a constant associated with self-generating continued fractions and Cahen's constant.

Original entry on oeis.org

6, 2, 9, 4, 6, 5, 0, 2, 0, 4, 5, 5, 1, 8, 6, 7, 7, 1, 8, 3, 1, 2, 9, 4, 2, 2, 9, 1, 0, 7, 2, 3, 2, 1, 2, 2, 6, 9, 3, 5, 3, 0, 0, 6, 9, 2, 3, 9, 0, 8, 8, 0, 5, 6, 1, 7, 5, 7, 0, 4, 5, 6, 1, 3, 2, 9, 8, 3, 4, 7, 4, 4, 3, 6, 1, 7, 3, 6, 2, 4, 9, 1, 9, 5, 3, 9, 9, 8, 8, 7, 7, 9, 4, 0, 7, 3, 7, 3, 9, 6

Offset: 0

Jean-François Alcover, May 21 2014

This constant is known to be transcendental.

Called the "Davison-Shallit constant" by Finch (2003) and Sondow (2021). - Amiram Eldar, Mar 19 2024

			0.62946502045518677183129422910723212269353...

Steven R. Finch, Mathematical Constants, Cambridge University Press, 2003, Section 6.7, p. 435.

Eugène Cahen, Note sur un développement des quantités numériques, qui présente quelque analogie avec celui en fractions continues, Nouvelles Annales de Mathématiques, Vol. 10 (1891), pp. 508-514. In French.
J. L. Davison and Jeffrey O. Shallit, Continued Fractions for Some Alternating Series, Monatshefte für Mathematik, Vol. 111 (1991), pp. 119-126; alternative link.
Jonathan Sondow, Irrationality and Transcendence of Alternating Series via Continued Fractions, in: A. Bostan and K. Raschel (eds.), Transcendence in Algebra, Combinatorics, Geometry and Number Theory, TRANS 2019. Springer Proceedings in Mathematics & Statistics, Vol. 373, Springer, Cham, 2021; arXiv preprint, arXiv:2009.14644 [math.NT], 2020.
Eric Weisstein's MathWorld, Cahen's constant.
Wikipedia, Cahen's constant.
Index entries for transcendental numbers.

Cf. A006277, A006279, A006280, A006281, A118227, A123180.

digits = 100; Clear[q, s]; q[n_] := q[n] = q[n - 2]*(q[n-1] + 1); q[0] = q[1] = 1; s[k_] := s[k] = Sum[(-1)^j/(q[j]*q[j+1]), {j, 0, k}] // N[#, digits+5]&; s[dk = 5]; s[k = 2*dk]; While[RealDigits[s[k], 10, digits] != RealDigits[s[k - dk], 10, digits], Print["k = ", k]; k = k + dk]; RealDigits[s[k], 10, digits] // First

Equals Sum_{k>=0} (-1)^k/(A006277(k)*A006277(k+1)). - Amiram Eldar, Mar 19 2024

cp's OEIS Frontend

A118227 Decimal expansion of Cahen's constant.

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A006280 Partial quotients in continued fraction expansion of Cahen's constant.

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A006281 Partial quotients in continued fraction expansion of 2C-1, where C is Cahen's constant.

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A123180 Even positions of Sylvester's sequence A000058; the denominators of the (greedy) Egyptian fraction expansion of Cahen's constant.

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A242724 Decimal expansion of a constant associated with self-generating continued fractions and Cahen's constant.

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