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A011900 a(n)=6*a(n-1)-a(n-2)-2, with a(0)=1, a(1)=3. 15
1, 3, 15, 85, 493, 2871, 16731, 97513, 568345, 3312555, 19306983, 112529341, 655869061, 3822685023, 22280241075, 129858761425, 756872327473, 4411375203411, 25711378892991, 149856898154533, 873430010034205 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Members of Diophantine pairs.

Solution to b(b-1) = 2a(a-1) in natural numbers; a = a(n), b = b(n) = A046090(n).

REFERENCES

Mario Velucchi "The Pell's equation ... an amusing application" in Mathematics and Informatics Quarterly, to appear 1997.

LINKS

Table of n, a(n) for n=0..20.

Index to sequences with linear recurrences with constant coefficients, signature (7,-7,1).

FORMULA

a(n)= (A001653(n)+1)/2.

a(n)=(((1+Sqrt(2))^(2*n-1)-(1-Sqrt(2))^(2*n-1))/Sqrt(8)+1)/2.

a_n = 7[a_(n-1) - a_(n-2)] + a_(n-3); a_(1) = 1, a_(2) = 3, a_(3) = 15. Also a(n) = 1/2 + ( (1-sqrt(2))/(-4*sqrt(2)) )*(3-2*sqrt(2))^n + ( (1+sqrt(2))/(4*sqrt(2)) )*(3+2*sqrt(2))^n. - Antonio Alberto Olivares, Dec 23 2003

Sqrt(2) = Sum_{n=0..inf} 1/a(n); a(n)=a(n-1)+floor(1/(Sqrt(2)-Sum_{k=0..n-1}1/a(k))) (n>0) with a(0)=1. - Paul D. Hanna, Jan 25 2004

For n>k, a(n+k)=A001541(n)*A001653(k)-A053141(n-k-1); e.g. 493=99*5-2. For n<=k, a(n+k)=A001541(n)*A001653(k)-A053141(k-n); e.g. 85=3*29-2 - Charlie Marion, Oct 18 2004

a(n+1)=3*a(n)-1+(8*a(n)^2-8*a(n)+1)^0.5, a(1)=1. - Richard Choulet, Sep 18 2007

a(n+1) = a(n) * (a(n) + 2) / a(n-1) for n>=1 with a(0)=1. [From Paul D. Hanna, Apr 08 2012]

G.f.: (1-4*x+x^2)/((1-x)*(1-6*x+x^2)). [From R. J. Mathar, Oct 26 2009]

PROG

(PARI) {a(n)=if(n<0, 0, if(n==0, 1, if(n==1, 3, a(n-1)*(a(n-1)+2)/a(n-2))))} /* Paul D. Hanna, Apr 08 2012 */

CROSSREFS

Cf. A001653, A046090.

Sequence in context: A202336 A093593 A212201 * A118342 A084209 A182016

Adjacent sequences:  A011897 A011898 A011899 * A011901 A011902 A011903

KEYWORD

nonn,easy

AUTHOR

Mario Velucchi (mathchess(AT)velucchi.it)

EXTENSIONS

More terms and comments from Wolfdieter Lang

STATUS

approved

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Last modified October 31 03:51 EDT 2014. Contains 248845 sequences.