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 A178000 Largest n-digit prime with the maximum number of digits equal to 2. 2
 2, 29, 229, 2221, 22229, 922223, 9222229, 22222223, 222222227, 7222222229, 22222222223, 522222222229, 9222222222229, 22222222222229, 222222222222227, 9222222222222227, 72222222222222221, 222222222222222221 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Select first for most 2's, then take the largest. In more detail: To get a(n), look at the list of all the n-digit primes. Suppose k is the maximum number of 2's of any number on the list. Throw out any prime on the list that does not contain k 2's. Then a(n) = maximal number that is left on the list. - N. J. A. Sloane, Mar 20 2018 For n <= 1000, a(n) has at most two non-2's.  What is the first n for which it has more than two? - Robert Israel, Mar 20 2018 LINKS Robert Israel, Table of n, a(n) for n = 1..999 MAPLE # This program will return FAIL if a(n) has more than two digits <> 2. f:= proc(n)    local k1, d2, k2, t;    for k1 in [9, 7, 3, 1] do if isprime(2/9*(10^n-1)+k1-2) then return 2/9*(10^n-1)+k1-2 fi od;    for d2 from n to 2 by -1 do      for k2 in [9, 8, 7, 6, 5, 4, 3] do        for k1 in [9, 7, 3, 1] do          t:= 2/9*(10^(n)-1)+(k2-2)*10^(d2-1) + k1-2;          if isprime(t) then return t fi;   od od od:   FAIL end proc: f(1):= 2: seq(f(n), n=1..30); # Robert Israel, Mar 20 2018 CROSSREFS Cf. A037057, A099656, A177999. Sequence in context: A244599 A261268 A020460 * A036939 A342962 A342971 Adjacent sequences:  A177997 A177998 A177999 * A178001 A178002 A178003 KEYWORD nonn,base AUTHOR Lekraj Beedassy, May 17 2010 STATUS approved

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Last modified August 16 22:14 EDT 2022. Contains 356169 sequences. (Running on oeis4.)