Truncatable prime

In number theory, a left-truncatable prime is a prime number which, in a given base, contains no 0, and if the leading ("left") digit is successively removed, then all resulting numbers are prime. For example 9137, since 9137, 137, 37 and 7 are all prime. Decimal representation is often assumed and always used in this article.

A right-truncatable prime is a prime which remains prime when the last ("right") digit is successively removed. For example 7393, since 7393, 739, 73, 7 are all prime.

There are exactly 4260 decimal left-truncatable primes::2, 3, 5, 7, 13, 17, 23, 37, 43, 47, 53, 67, 73, 83, 97, 113, 137, 167, 173, 197, 223, 283, 313, 317, 337, 347, 353, 367, 373, 383, 397, 443, 467, 523, 547, 613, 617, 643, 647, 653, 673, 683, 743, 773, 797, 823, 853, 883, 937, 947, 953, 967, 983, 997, 1223, 1283, 1367 ... OEIS|id=A024785The largest is the 24-digit 357686312646216567629137.

There are 83 right-truncatable primes. The complete list::2, 3, 5, 7, 23, 29, 31, 37, 53, 59, 71, 73, 79, 233, 239, 293, 311, 313, 317, 373, 379, 593, 599, 719, 733, 739, 797, 2333, 2339, 2393, 2399, 2939, 3119, 3137, 3733, 3739, 3793, 3797, 5939, 7193, 7331, 7333, 7393, 23333, 23339, 23399, 23993, 29399, 31193, 31379, 37337, 37339, 37397, 59393, 59399, 71933, 73331, 73939, 233993, 239933, 293999, 373379, 373393, 593933, 593993, 719333, 739391, 739393, 739397, 739399, 2339933, 2399333, 2939999, 3733799, 5939333, 7393913, 7393931, 7393933, 23399339, 29399999, 37337999, 59393339, 73939133 OEIS|id=A024770The largest is the 8-digit 73939133. All primes above 5 end with digit 1, 3, 7 or 9, so a right-truncatable prime can only contain those digits after the leading digit.

There are 15 primes which are both left-truncatable and right-truncatable. They have been called two-sided primes. The complete list::2, 3, 5, 7, 23, 37, 53, 73, 313, 317, 373, 797, 3137, 3797, 739397 (OEIS2C|id=A020994)

While the primality of a number does not depend on the numeral system used, truncatable primes are defined only in relation with a given base. A variation involves removing 2 or more decimal digits at a time. This is mathematically equivalent to using base 100 or a larger power of 10, with the restriction that base 10ⁿ digits must be at least 10ⁿ⁻¹, in order to match a decimal n-digit number with no leading 0.

ee also

*Permutable prime

References

*MathWorld|title=Truncatable Prime|urlname=TruncatablePrime
*Caldwell, Chris, [http://primes.utm.edu/glossary/page.php?sort=LeftTruncatablePrime "left-truncatable prime"] and [http://primes.utm.edu/glossary/page.php?sort=RightTruncatablePrime "right-truncatable primes"] , at the Prime Pages glossary.
*Rivera, Carlos, [http://www.primepuzzles.net/puzzles/puzz_002.htm Problems & Puzzles: Puzzle 2.- Prime strings]