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A346022
Primes that are the first in a run of exactly 2 emirps.
8
13, 31, 337, 701, 761, 937, 983, 1151, 1279, 1831, 1933, 3191, 3803, 3851, 3911, 7043, 7219, 7457, 7523, 7643, 9127, 9161, 9241, 9437, 9521, 9547, 9601, 9871, 9931, 10007, 10151, 10247, 10487, 10639, 10853, 10889, 11071, 11657, 11833, 12071, 12547, 12689
OFFSET
1,1
COMMENTS
There are large gaps in this sequence because all terms need to begin with 1, 3, 7, or 9 otherwise the reversal is composite.
EXAMPLE
a(2) = 31 because of the four consecutive primes 29, 31, 37, 41 only 31, 37 are emirps.
PROG
(Python)
from sympy import isprime, nextprime
def isemirp(p): s = str(p); return s != s[::-1] and isprime(int(s[::-1]))
def aupto(limit):
alst, pvec, evec, p = [], [2, 3, 5, 7], [0, 0, 0, 0], 11
while pvec[1] <= limit:
if evec == [0, 1, 1, 0]: alst.append(pvec[1])
pvec = pvec[1:] + [p]; evec = evec[1:] + [isemirp(p)]; p = nextprime(p)
return alst
print(aupto(12689)) # Michael S. Branicky, Jul 04 2021
CROSSREFS
Subsequence of A006567 (emirps).
Sequence in context: A299888 A023304 A247837 * A227172 A180757 A214488
KEYWORD
nonn,base
AUTHOR
Lars Blomberg, Jul 01 2021
STATUS
approved