A056730 Palindromic primes with just two distinct digits.

101, 131, 151, 181, 191, 313, 353, 373, 383, 727, 757, 787, 797, 919, 929, 11311, 11411, 13331, 15551, 16661, 18181, 19991, 32323, 33533, 35353, 72227, 72727, 74747, 75557, 76667, 77377, 77477, 77977, 78787, 78887, 79997, 94949, 95959

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

f:= proc(d) local d1, d2, L, cand, C1, C2, C3, a, b, n, Cands;
    Cands:= NULL;
    d1:= (d+1)/2;
    d2:= d-d1;
    for b in [1, 3, 7, 9] do
      for a in {$0..9} minus {b} do
        for n from 2^(d1-1) to 2^d1-2 do
          L:= convert(n, base, 2);
          C1:= a*(10^d-1)/9;
          C2:= (b-a)*add(L[i]*10^(i+d2-1), i=1..d1);
          C3:= (b-a)*add(L[i]*10^(d1-i), i=2..d1);
          cand:= C1+C2+C3;
          if isprime(cand) then Cands:= Cands, cand; fi
    od od od;
    sort([Cands])
end proc:
map(op, [seq(f(d), d=3..7, 2)]); # Robert Israel, Sep 09 2018

Mathematica

Select[ Range[ 1, 3 10^6, 2 ], PrimeQ[ # ] && Length[ Union[ RealDigits[ # ][ [ 1 ] ] ] ] == 2 && RealDigits[ # ][ [ 1 ] ] == Reverse[ RealDigits[ # ][ [ 1 ] ] ] & ]

Crossrefs

Intersection of A002385 and A031955.

Sequence in context: A319049 A052086 A154270 * A077798 A089360 A056728

Adjacent sequences: A056727 A056728 A056729 * A056731 A056732 A056733

Keyword

nonn,base

Author

Robert G. Wilson v, Aug 11 2000

Status

approved