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A018800 Smallest prime that begins with n. 16
11, 2, 3, 41, 5, 61, 7, 83, 97, 101, 11, 127, 13, 149, 151, 163, 17, 181, 19, 2003, 211, 223, 23, 241, 251, 263, 271, 281, 29, 307, 31, 3203, 331, 347, 353, 367, 37, 383, 397, 401, 41, 421, 43, 443, 457, 461, 47, 487, 491, 503, 5101, 521, 53, 541, 557, 563, 571, 587, 59 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: If a(n) = (n concatenated with k) then k < n. - Amarnath Murthy, May 01 2002

a(n) always exists. Proof. Suppose n is L digits long, and consider the numbers between n*10^B and n*10^B+10^C, where B>C are both large compared with L. All such numbers begin with the digits of n. Using the upper and lower bounds on pi(x) from Theorem 1 of Rosser and Schoenfeld, it follows that for sufficiently large B and C, at least one of these numbers is a prime. QED - N. J. A. Sloane, Nov 14 2014

LINKS

Paolo P. Lava and T. D. Noe, Table of n, a(n) for n = 1..1000 (first 100 terms from Paolo P. Lava)

J. Barkley Rosser and Lowell Schoenfeld, Approximate formulas for some functions of prime numbers, Illinois J. Math. 6 (1962), pp. 64-94.

Index entries for primes involving decimal expansion of n

FORMULA

a(n) = prime(A085608(n)). - Michel Marcus, Oct 19 2013

MAPLE

f:= proc(n) local x0, d, r, y;

   if isprime(n) then return(n) fi;

   for d from 1 do

     x0:= n*10^d;

     for r from 1 to 10^d-1 by 2 do

       if isprime(x0+r) then

          return(x0+r)

       fi

     od

   od

end proc:

seq(f(n), n=1..100); # Robert Israel, Dec 23 2014

MATHEMATICA

Table[Function[d, FromDigits@ SelectFirst[ IntegerDigits@ Prime@ Range[10^4], Length@ # >= Length@ d && Take[#, Length@ d] == d &]][ IntegerDigits@ n], {n, 59}] (* Michael De Vlieger, May 24 2016, Version 10 *)

PROG

(Haskell)

import Data.List (isPrefixOf, find); import Data.Maybe (fromJust)

a018800 n = read $ fromJust $

            find (show n `isPrefixOf`) $ map show a000040_list :: Int

-- Reinhard Zumkeller, Jul 01 2015

(PARI) a(n{, base=10}) = for (l=0, oo, forprime (p=n*base^l, (n+1)*base^l-1, return (p))) \\ Rémy Sigrist, Jun 11 2017

(Python)

from sympy import isprime

def a(n):

    if isprime(n): return n

    pow10 = 10

    while True:

        t, maxt = n * pow10 + 1, (n+1) * pow10

        while t < maxt:

            if isprime(t): return t

            t += 2

        pow10 *= 10

print([a(n) for n in range(1, 60)]) # Michael S. Branicky, Nov 02 2021

CROSSREFS

Cf. A030665, A068164, A068695, A062584, A088781, A085608.

A164022 is the base-2 analog.

Cf. also A258337.

Row n=1 of A262369.

Sequence in context: A077549 A089356 A113616 * A258337 A089566 A010191

Adjacent sequences:  A018797 A018798 A018799 * A018801 A018802 A018803

KEYWORD

nonn,base

AUTHOR

David W. Wilson

STATUS

approved

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Last modified May 20 06:37 EDT 2022. Contains 353852 sequences. (Running on oeis4.)