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
Giovanni Resta, Table of n, a(n) for n = 1..10000
FORMULA
From M. F. Hasler, Dec 21 2019: (Start)
a(n) = a(n-4) + 10 for 4 < n < 41, i.e., 20 < a(n) < 110; a(n) = a(n-25) 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 1-digit 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( k-Reverse(Intseq(k))[1]*10^(#Intseq(k)-1 ))]; // Marius A. Burtea, Dec 21 2019
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Jonathan Vos Post, May 26 2013
STATUS
approved