A013705 - OEIS

A013705

Decimal expansion of 4*Sum_{k=1..500000} (-1)^(k-1)/(2k-1).

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

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OFFSET

1,1

COMMENTS

An approximation to Pi.

A case of "high precision fraud": curiously, among the first 40 digits, only 4 are wrong (in positions 7, 18, 19 and 30). - Jean-François Alcover, Apr 23 2013

This result arises because the sum is Pi - 2*10^-6 + 2*10^-18 - 10^-29 + 122*10^-42 - ... - Jon E. Schoenfield, Mar 11 2018

LINKS

Table of n, a(n) for n=1..105.

J. M. Borwein and P. B. Borwein, Strange series and high precision fraud, Amer. Math. Monthly 99, 7 (Aug. 1992), 622-640.

J. M. Borwein, P. B. Borwein and K. Dilcher, Pi, Euler numbers and asymptotic expansions, Amer. Math. Monthly, 96 (1989), 681-687.

J. M. Borwein and R. M. Corless, Review of "An Encyclopedia of Integer Sequences" by N. J. A. Sloane and Simon Plouffe, SIAM Review, 38 (1996), 333-337.

EXAMPLE

3.1415906535897932404626433832695028841972913993751030509749446933498...

PROG

(PARI) 4*sum(k=1, 500000, (-1.)^(k-1)/(2*k-1)) \\ Michel Marcus, Mar 11 2018

CROSSREFS

Cf. A000796, A216542, A013706, A216543, A216544, A216545, A013705, A216546, A216547, A216548.

Cf. also A195793.

Sequence in context: A324242 A216543 A152040 * A358989 A216548 A087478

Adjacent sequences: A013702 A013703 A013704 * A013706 A013707 A013708

KEYWORD

cons,nonn

AUTHOR

N. J. A. Sloane

EXTENSIONS

a(78)-a(80) corrected and more digits from Jon E. Schoenfield, Mar 11 2018

STATUS

approved