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 A241206 Greatest n-digit prime having at least n-1 identical digits. 7
 7, 97, 997, 9949, 99991, 999979, 9999991, 99999989, 999999929, 9999999929, 99999999599, 999999999989, 9999999999799, 99999999999959, 999999999999989, 9999999999999199, 99999999999999997, 999999999999999989, 9999999999999999919, 99999999999999999989 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Not the same as A069661 (Smallest n-digit prime with maximum digit sum). For example, A069661(10) = 9899989999 with only n-2 = 8 identical digits. Conjecture: each term consists of at least n-1 digits 9 and no digit 0. - Chai Wah Wu, Dec 10 2015 LINKS Chai Wah Wu, Table of n, a(n) for n = 1..1000 (corrected by Georg Fischer, Jan 20 2019) MAPLE with(numtheory):lst:={}:nn:=30:kk:=0:T:=array(1..nn):U:=array(1..20):    for n from 2 to nn do:      for i from 1 to n do:      T[i]:=9:      od:      ii:=0:         for j from 1 to n while(ii=0)do:         for k from 9 by -1 to 0 while(ii=0)do:         T[n-j+1]:=k:s:=sum('T[i]*10^(n-i)', 'i'=1..n):          if type(s, prime)=true and length(s)=n          then          ii:=1: kk:=kk+1:U[kk]:=s:          else          T[n-j+1]:=9:          fi:        od:      od:     od :      print(U) : MATHEMATICA Table[SelectFirst[Reverse@ Prime@ Range[PrimePi[10^(n - 1)] + 1, PrimePi[10^n - 1]], Max@ DigitCount@ # >= (n - 1) &], {n, 2, 8}] (* WARNING: the following assumes the conjecture is true WARNING *) Table[SelectFirst[Select[Reverse@ Union@ Map[FromDigits, Join @@ Map[Permutations[Append[Table[9, {n - 1}], #]] &, Range[0, 9]]], PrimeQ@ # && IntegerLength@ # == n &], Max@ DigitCount@ # >= (n - 1) &], {n, 2, 20}] (* Michael De Vlieger, Dec 10 2015, Version 10 *) PROG (Python) from __future__ import division from sympy import isprime def A241206(n):     for i in range(9, 0, -1):         x = i*(10**n-1)//9         for j in range(n-1, -1, -1):             for k in range(9-i, -1, -1):                 y = x + k*(10**j)                 if isprime(y):                     return y         for j in range(n):             for k in range(1, i+1):                 if j < n-1 or k < i:                     y = x-k*(10**j)                     if isprime(y):                         return y # Chai Wah Wu, Dec 29 2015 CROSSREFS Cf. A069661, A241100. Sequence in context: A146758 A114019 A178007 * A127892 A125590 A068694 Adjacent sequences:  A241203 A241204 A241205 * A241207 A241208 A241209 KEYWORD nonn,base AUTHOR Michel Lagneau, Apr 17 2014 EXTENSIONS a(1) added - N. J. A. Sloane, Dec 29 2015 STATUS approved

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Last modified January 19 18:13 EST 2020. Contains 331051 sequences. (Running on oeis4.)