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
KEYWORD
nonn,base
AUTHOR
Lars Blomberg, Jul 01 2021
STATUS
approved