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 A061037 Numerator of 1/4 - 1/n^2. 75
 0, 5, 3, 21, 2, 45, 15, 77, 6, 117, 35, 165, 12, 221, 63, 285, 20, 357, 99, 437, 30, 525, 143, 621, 42, 725, 195, 837, 56, 957, 255, 1085, 72, 1221, 323, 1365, 90, 1517, 399, 1677, 110, 1845, 483, 2021, 132, 2205, 575, 2397, 156, 2597, 675 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS From Balmer spectrum of hydrogen. Wavelengths in hydrogen spectrum are given by Rydberg's formula 1/wavelength = constant*(1/m^2 - 1/n^2). a(n)= mix (A142705=A026741(n)*A026741(n+2)) , (A078371=A005408(n)*A005408(n+2)). Note A026741=mix A001477 , A005408. a(4n)=A001477(n)*A001477(n+1). - Paul Curtz, Aug 27 2009 a(-2)=0, a(-1)=a(1)=-3. a(n) = 3*a(n-4) - 3*a(n-8) + a(n-12). - R. J. Mathar This recurrence is also valuable for a(n) differences and A061038(n+2) and its differences. Can be thought of as 4 interlocking sequences, each of the form a(n) = 3a(n - 1) - 3a(n - 2) + a(n - 3). - Charles R Greathouse IV, May 27 2011 REFERENCES J. E. Brady and G. E. Humiston, General Chemistry, 3rd. ed., Wiley; p. 78. LINKS Harry J. Smith, Table of n, a(n) for n=2..1000 J. J. O'Connor and E. F. Robertson, Johannes Robert Rydberg Wikipedia, Balmer series Index entries for linear recurrences with constant coefficients, signature (0,0,0,3,0,0,0,-3,0,0,0,1). FORMULA G.f.: x^2(-3x^11-x^10-3x^9+14x^7+6x^6+30x^5+2x^4+21x^3+3x^2+5x)/(1-x^4)^3. a(4n+2) = n(n+1), a(2n+3) = (2n+1)(2n+5), a(4n+4) = (2n+1)(2n+3). - Ralf Stephan, Jun 10 2005 a(n+2) = A060819(n)*A060819(n+4). a(n) = (n^2-4)*(3*I^n+3*(-I)^n-27*(-1)^n+37)/64, where I is the imaginary unit. - Bruno Berselli, Feb 10 2011 a(n+2)=n*(n+4)/(period 4: 16, 1, 4, 1 =A146160(n)) = A028347(n+2) / A146160(n). - Paul Curtz, Mar 24 2011 [edited by Franklin T. Adams-Watters, Mar 25 2011] a(n) = (n^2-4) / gcd(4*n^2, (n^2-4)). - Colin Barker, Jan 13 2014 MATHEMATICA f[n_] := n/GCD[n, 4]; Array[f[#] f[# + 4] &, 51, 0] f[n_] := Numerator[(n - 2) (n + 2)/(4 n^2)]; Array[f, 51, 2] (* Or *) a[n_] := 3 a[n - 4] - 3 a[n - 8] + a[n - 12]; a[1] = -3; a[2] = 0; a[3] = 5; a[4] = 3; a[5] = 21; a[6] = 2; a[7] = 45; a[8] = 15; a[9] = 77; a[10] = 6; a[11] = 117; a[12] = 35; Array[a, 51, 2] (* Robert G. Wilson v *) Numerator[1/4-1/Range[2, 60]^2] (* Harvey P. Dale, Aug 18 2011 *) PROG (PARI) { for (n=2, 1000, write("b061037.txt", n, " ", numerator(1/4 - 1/n^2)) ) } \\ Harry J. Smith, Jul 17 2009 (MAGMA) [ Numerator(1/4-1/n^2): n in [2..52] ]; // Bruno Berselli, Feb 10 2011 (Haskell) import Data.Ratio ((%), numerator) a061037 n = numerator (1%4 - 1%n^2)  -- Reinhard Zumkeller, Dec 17 2011 CROSSREFS Cf. A061035-A061050, A126252, A028347. Sequence in context: A256565 A248256 A049457 * A070262 A171621 A084183 Adjacent sequences:  A061034 A061035 A061036 * A061038 A061039 A061040 KEYWORD nonn,frac,nice,easy AUTHOR N. J. A. Sloane, May 26 2001 STATUS approved

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