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 A066770 5^n sin(2n arctan(1/2)) or numerator of tan(2n arctan(1/2)). 7
 4, 24, 44, -336, -3116, -10296, 16124, 354144, 1721764, 1476984, -34182196, -242017776, -597551756, 2465133864, 29729597084, 116749235904, -42744511676, -3175197967656, -17982575014036, -28515500892816, 278471369994004, 2383715742284424, 7340510203856444 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 430-433. LINKS J. M. Borwein and R. Girgensohn, Addition theorems and binary expansions, Canadian J. Math. 47 (1995) 262-273. E. Eckert, The group of primitive Pythagorean triangles, Mathematics Magazine 57 (1984) 22-27. S. R. Finch, Plouffe's Constant Simon Plouffe, The Computation of Certain Numbers Using a Ruler and Compass, J. Integer Seqs. Vol. 1 (1998), #98.1.3. Index entries for linear recurrences with constant coefficients, signature (6,-25). FORMULA G.f.: 4*x/(1-6*x+25*x^2). - Ralf Stephan, Jun 12 2003 a(n) = 5^n sin(2n arctan(1/2)). A recursive formula for T(n) = tan(2n arctan(1/2)) is T(n+1)=(4/3+T(n))/(1-4/3*T(n)). Unsigned a(n) is the absolute value of numerator of T(n). a(n) is the imaginary part of (2+I)^(2n) = sum(k=0, n, 2^(2*n-2*k-1)*(-1)^k*binomial(2*n, 2*k+1) ). - Benoit Cloitre, Aug 03 2002 a(n) = 6*a(n-1)-25*a(n-2), n>2. - Gary Detlefs, Dec 11 2010 a(n) = 5^n*sin(nx), where x = arcsin(4/5) = 0.927295218.. . - Gary Detlefs, Dec 11 2010 MAPLE a[1] := 4/3; for n from 1 to 40 do a[n+1] := (4/3+a[n])/(1-4/3*a[n]):od: seq(abs(numer(a[n])), n=1..40); # a[n]=tan(2n arctan(1/2)) MATHEMATICA Table[ 5^n*Sin[2*n*ArcCot[2]] // Simplify, {n, 1, 23}] (* Jean-François Alcover, Mar 04 2013 *) PROG (PARI) a(n)=imag((2+I)^(2*n)) CROSSREFS Cf. A066771, A000351 powers of 5 and also hypotenuse of right triangle with legs given by A066770 and A066771. Note that A066770, A066771 and A000351 are primitive Pythagorean triples with hypotenuse 5^n. The offset of A000351 is zero, but the offset is 1 for A066770, A066771. Sequence in context: A174178 A139245 A224242 * A080380 A039935 A090821 Adjacent sequences:  A066767 A066768 A066769 * A066771 A066772 A066773 KEYWORD sign,easy,frac AUTHOR Barbara Haas Margolius, (b.margolius(AT)csuohio.edu), Jan 17 2002 STATUS approved

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Last modified August 17 21:09 EDT 2018. Contains 313817 sequences. (Running on oeis4.)