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A057680 Self-locating strings within Pi: numbers n such that the string n is at position n (after the decimal point) in decimal digits of Pi. 17
1, 16470, 44899, 79873884, 711939213, 36541622473, 45677255610, 62644957128, 656430109694 (list; graph; refs; listen; history; text; internal format)



The total probability of finding a match of length "n" digits is exactly 0.9.  That is, we expect 0.9 matches with 1 digit, 0.9 matches with 2 digits, etc.  Increasing the number of digits by a factor of 10 means that we expect to find 0.9 new matches.  Increasing the search from 10^11 to 10^12 (which includes 10 times as much work) would thus only expect to find 0.9 new matches. - Alan Eliasen, May 01 2013

Consequently, with the second Borel-Cantelli lemma, the expected number of terms in this sequence is infinite with probability 1. (Of course the sequence is not random, but almost all of the sequences corresponding to randomly-chosen real numbers in place of Pi have infinitely many terms.) - Charles R Greathouse IV, Apr 29 2013

a(10) is greater than 10^12. - Alan Eliasen, Jun 17 2013

a(1) & a(5) are the first occurrences in Pi of their respective strings; a(2) & a(4) are the second occurrences; a(3) is the fourth occurrence. - Hans Havermann, Jul 27 2014


Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60.


Table of n, a(n) for n=1..9.

David G. Andersen, The Pi-Search Page.


1 is a term because 1 is the first digit after the decimal point.


StringsinPiAfterPoint[m_] := Module[{cc = 10^m + m, sol, aa}, sol = Partition[RealDigits[Pi, 10, cc] // First // Rest, m, 1]; Do[aa = FromDigits[sol[[i]]]; If[aa==i, Print[{i, aa}]], {i, Length[sol]}]; ] For example, StringsinPiAfterPoint[5] returns all 5-digit members of the sequence. - Colin Rose, Mar 15 2006


Cf. A000796, A057679, A109513, A064810.

Sequence in context: A168665 A283027 A031829 * A157796 A186848 A211841

Adjacent sequences:  A057677 A057678 A057679 * A057681 A057682 A057683




Mike Keith (domnei(AT)aol.com), Oct 19 2000


More terms from Colin Rose, Mar 15 2006

a(5) from Nathaniel Johnston, Nov 12 2010

a(6)-a(8) from Alan Eliasen, May 01 2013

a(9) from Alan Eliasen, Jun 06 2013



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Last modified October 20 02:54 EDT 2019. Contains 328244 sequences. (Running on oeis4.)