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 A072172 a(n) = (2*n+1)*5^(2*n+1). 5
 5, 375, 15625, 546875, 17578125, 537109375, 15869140625, 457763671875, 12969970703125, 362396240234375, 10013580322265625, 274181365966796875, 7450580596923828125, 201165676116943359375, 5401670932769775390625, 144354999065399169921875 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS J. Machin (died 1751) used Pi/4 = 4*Sum_{n=0..inf} (-1)^n/((2*n+1)*5^(2*n+1)) - Sum_{n=0..inf} (-1)^n/((2*n+1)*239^(2*n+1)) to calculate Pi to 100 decimal places. REFERENCES H. Doerrie, 100 Great Problems of Elementary Mathematics, Dover, NY, 1965, p. 73 LINKS Colin Barker, Table of n, a(n) for n = 0..700 Index entries for linear recurrences with constant coefficients, signature (50,-625). FORMULA From Colin Barker, Aug 25 2016: (Start) a(n) = 50*a(n-1) - 625*a(n-2) for n>1. G.f.: 5*(1+25*x)/(1-25*x)^2. (End) From Ilya Gutkovskiy, Aug 25 2016: (Start) E.g.f.: 5*(1 + 50*x)*exp(25*x). Sum_{n>=0} 1/a(n) = arctanh(1/5) = 0.2027325540540821... Sum_{n>=0} (-1)^n/a(n) = arctan(1/5) = A105532 (End) MAPLE seq((2*n+1)*5^(2*n+1), n=0..20); # G. C. Greubel, Aug 26 2019 MATHEMATICA Table[(2*n+1)*5^(2*n+1), {n, 0, 20}] (* G. C. Greubel, Aug 26 2019 *) PROG (PARI) Vec(5*(1+25*x)/(1-25*x)^2 + O(x^20)) \\ Colin Barker, Aug 25 2016 (PARI) vector(20, n, (2*n-1)*5^(2*n-1) ) \\ G. C. Greubel, Aug 26 2019 (Magma) [(2*n+1)*5^(2*n+1): n in [0..20]]; // G. C. Greubel, Aug 26 2019 (Sage) [(2*n+1)*5^(2*n+1) for n in (0..20)] # G. C. Greubel, Aug 26 2019 (GAP) List([0..20], n-> (2*n+1)*5^(2*n+1)); # G. C. Greubel, Aug 26 2019 CROSSREFS Cf. A072173. Cf. A157332. - Jaume Oliver Lafont, Mar 03 2009 Sequence in context: A215437 A098038 A354831 * A278364 A214008 A208094 Adjacent sequences: A072169 A072170 A072171 * A072173 A072174 A072175 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Jun 30 2002 STATUS approved

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Last modified June 25 13:13 EDT 2024. Contains 373705 sequences. (Running on oeis4.)