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A347343
Positive integers k such that k with the last digit repeated is prime.
2
1, 19, 21, 23, 27, 31, 43, 49, 57, 59, 67, 73, 81, 87, 91, 97, 103, 127, 139, 143, 149, 151, 169, 173, 177, 181, 187, 193, 199, 201, 209, 211, 231, 233, 237, 239, 241, 247, 263, 267, 269, 271, 277, 283, 299, 301, 329, 343, 349, 351, 353, 367, 373, 383, 387
OFFSET
1,2
LINKS
FORMULA
a(n) ~ n log n by the prime number theorem in arithmetic progressions. (These numbers are the primes mod 11, 33, 77, or 99 mod 100 with their last digit removed.) - Charles R Greathouse IV, Aug 27 2021
EXAMPLE
21 is a term because 211 is prime.
MATHEMATICA
upto=500; Select[Range[1, upto, 2], PrimeQ[FromDigits[Join[d=IntegerDigits[#], {Last[d]}]]]&]
PROG
(PARI) forprime(p=9, 1e4, if(p%100%11==0, print1(p\10", "))) \\ Charles R Greathouse IV, Aug 27 2021
(Python)
from sympy import isprime
def ok(n): return isprime(10*n + n%10)
print(list(filter(ok, range(388)))) # Michael S. Branicky, Aug 28 2021
CROSSREFS
Sequence in context: A089837 A020347 A334103 * A054864 A158232 A178889
KEYWORD
nonn,base,easy
AUTHOR
Paolo Xausa, Aug 27 2021
STATUS
approved