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 A003618 Largest n-digit prime. (Formerly M4452) 42
 7, 97, 997, 9973, 99991, 999983, 9999991, 99999989, 999999937, 9999999967, 99999999977, 999999999989, 9999999999971, 99999999999973, 999999999999989, 9999999999999937, 99999999999999997, 999999999999999989, 9999999999999999961, 99999999999999999989 (list; graph; refs; listen; history; text; internal format)
 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, -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 Cf. A003617, A033874, A114429 (largest n-digit twin prime). Sequence in context: A125590 A068694 A158579 * A173833 A093172 A074110 Adjacent sequences: A003615 A003616 A003617 * A003619 A003620 A003621 KEYWORD nonn,nice,base AUTHOR EXTENSIONS More terms from Stefan Steinerberger, Apr 08 2006 STATUS approved

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Last modified March 28 03:48 EDT 2023. Contains 361577 sequences. (Running on oeis4.)