OFFSET
1,1
COMMENTS
Since 10^n - 1 is always a multiple of 9, one could be tempted to think that 9 is the least frequently occurring least significant digit in terms of this sequence. - Alonso del Arte, Dec 03 2017
The occurrences of least significant digits in the first 8000 terms (see A033874) are 1: 2028, 3: 2032, 7: 2014, and 9: 1926. - Giovanni Resta, Mar 16 2020
REFERENCES
O'Hara, J. Rec. Math., 22 (1990), Table on page 278.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..1000 (terms 1..200 from T. D. Noe. Jon E. Schoenfield verified that terms 1..200 are indeed primes, Feb 02 2009)
Eric Weisstein's World of Mathematics, Previous Prime
R. G. Wilson, v., Extract from letter to N. J. A. Sloane, May 20 1994, with annotated scanned copy of page 278 of O'Hara article.
EXAMPLE
No power of 10 is prime.
9 = 3^2, 8 = 2^3 but 7 is prime, so a(1) = 7.
99 = 3^2 * 11 but 97 is prime, so a(2) = 97.
999 = 3^3 * 37 but 997 is prime, so a(3) = 997.
9999 = 3^2 * 11 * 101, 9997 = 13 * 769, 9995 = 5 * 1999, 9993 = 3 * 3331, 9991 = 97 * 103, ..., 9975 = 5^2 * 399, but 9973 is prime, so a(4) = 9973.
MAPLE
a:= n-> prevprime(10^n):
seq(a(n), n=1..20); # Alois P. Heinz, Feb 11 2021
MATHEMATICA
NextPrime[10^Range[20], -1] (* Harvey P. Dale, Feb 03 2011 *)
PROG
(PARI) a(n)=precprime(10^n) \\ Charles R Greathouse IV, Jul 19 2011
(Magma) [PreviousPrime(10^n): n in [1..20]]; // Vincenzo Librandi, Sep 13 2016
(Python)
from sympy import prevprime
print([prevprime(10**n) for n in range(1, 21)]) # Michael S. Branicky, Feb 11 2021
CROSSREFS
KEYWORD
nonn,nice,base
AUTHOR
EXTENSIONS
More terms from Stefan Steinerberger, Apr 08 2006
STATUS
approved