The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A125590 Largest n-digit base-10 deletable prime. 1
 7, 97, 997, 9973, 99929, 999907, 9999907, 99999307, 999996671, 9999996073, 99999966307, 999999908773, 9999999710639, 99999999697769, 999999997160639, 9999999996977699 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A prime p is a base-b deletable prime if when written in base b it has the property that removing some digit leaves either the empty string or another deletable prime. "Digit" means digit in base b. Deleting a digit cannot leave any leading zeros in the new string. For example, deleting the 2 in 2003 to obtain 003 is not allowed. REFERENCES C. Caldwell, Truncatable primes, J. Recreational Math., 19:1 (1987) 30-33. [Discusses left truncatable primes, right truncatable primes and deletable primes.] LINKS I. O. Angell and H. J. Godwin, On Truncatable Primes, Math. Comput. 31, 265-267, 1977. C. Caldwell, Deletable primes Prime Curios, A 300-digit example Prime Puzzles, Puzzle 138: Deletable Primes [Includes a 500-digit example] EXAMPLE 99929 -> 9929 -> 929 -> 29 -> 2. MATHEMATICA b = 10; a = {7}; d = {2, 3, 5, 7}; For[n = 2, n <= 5, n++,   p = Select[Range[b^(n - 1), b^n - 1], PrimeQ[#] &];   For[i = 1, i <= Length[p], i++,    c = IntegerDigits[p[[i]], b];    For[j = 1, j <= n, j++,     t = Delete[c, j];     If[t[[1]] == 0, Continue[]];     If[MemberQ[d, FromDigits[t, b]], AppendTo[d, p[[i]]]; Break[]]]];   AppendTo[a, Last[d]]]; a (* Robert Price, Nov 13 2018 *) CROSSREFS Cf. A080608, A096243, A096246, A125589. Sequence in context: A178007 A241206 A127892 * A068694 A158579 A003618 Adjacent sequences:  A125587 A125588 A125589 * A125591 A125592 A125593 KEYWORD nonn,base,more AUTHOR N. J. A. Sloane, Jan 07 2007 EXTENSIONS a(6)-a(8) from Michael Kleber, Jan 08 2007 Extended through a(17) by Joshua Zucker STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 21 17:36 EDT 2021. Contains 343156 sequences. (Running on oeis4.)