

A226099


Positive integers that yield a prime when their most significant (i.e., leftmost) decimal digit is removed.


4



12, 13, 15, 17, 22, 23, 25, 27, 32, 33, 35, 37, 42, 43, 45, 47, 52, 53, 55, 57, 62, 63, 65, 67, 72, 73, 75, 77, 82, 83, 85, 87, 92, 93, 95, 97, 102, 103, 105, 107, 111, 113, 117, 119, 123, 129, 131, 137, 141, 143, 147, 153, 159, 161, 167, 171, 173, 179, 183, 189, 197, 202, 203, 205, 207, 211, 213, 217
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OFFSET

1,1


COMMENTS

Terms < 110 are the same as in A260181, numbers whose last digit is prime.  M. F. Hasler, Dec 20 2019
These are numbers with decimal expansion of the form k = xp where p is a prime and x is a single digit. Whether or not the number k itself is a prime is irrelevant.  N. J. A. Sloane, Dec 21 2019


LINKS



FORMULA

a(n) = a(n4) + 10 for 4 < n < 41, i.e., 20 < a(n) < 110; a(n) = a(n25) for 61 < n < 287, i.e., 200 < a(n) < 1100, etc. (End)


EXAMPLE

a(1) = 12 because when its most significant (or leftmost) digit (1) is removed, the remaining number 2 is prime, and it is the least such number.
102, 103, 105 and 107 are in the sequence because if the first digit is dropped, what is left is a 1digit prime with a leading digit '0'.


MATHEMATICA

Select[Range@ 300, PrimeQ@ FromDigits@ Rest@ IntegerDigits@ # &] (* Giovanni Resta, Dec 20 2019 *)


PROG

(PARI) select( is(n)=isprime(n%10^logint(n+!n, 10)), [0..222]) \\ M. F. Hasler, Dec 20 2019
(Magma) [k:k in [1..220] IsPrime( kReverse(Intseq(k))[1]*10^(#Intseq(k)1 ))]; // Marius A. Burtea, Dec 21 2019


CROSSREFS



KEYWORD

nonn,base,easy


AUTHOR



STATUS

approved



