A056728 Palindromic primes using only two distinct digits and only the exterior digit is different.

101, 131, 151, 181, 191, 313, 353, 373, 383, 727, 757, 787, 797, 919, 929, 13331, 15551, 16661, 19991, 72227, 75557, 76667, 78887, 79997, 1333331, 1444441, 1777771, 3222223, 3444443, 7666667, 9222229, 9888889

Offset: 1

Author

Robert G. Wilson v, Aug 11 2000

Keywords

• nonn
• base

Comments

Primes of the form a*(10^c + 1) + b*(10^c - 10)/9 for 1<=a<=9, 0<=b<=9, c >= 2, a <> b. - Robert Israel, Nov 02 2014

Links

• Robert Israel, Table of n, a(n) for n = 1..127
• Patrick De Geest, PDP Reference Table.
• Makoto Kamada, Plateau and depression numbers of the form ABB...BBA.

Programs

Maple

f:= (a,b,n) -> a*(10^n + 1) + b*(10^n - 10)/9:
select(isprime,[seq(seq(seq(f(a,b,n),b={$0..9} minus {a}),a=1..9),n=2..8)]);
# Robert Israel, Nov 02 2014

Mathematica

Select[Union[Flatten[Table[FromDigits[Join[{a},PadRight[{},n,b],{a}]],{n,1,7,2},{b,0,9},{a,{1,3,7,9}}]]],PrimeQ] (* Harvey P. Dale, Mar 07 2015 *)

Extensions

Links added by Patrick De Geest, Nov 02 2014