A182721 Mountain emirps.

1231, 1321, 1381, 1471, 1741, 1831, 12491, 12641, 12841, 13591, 13751, 13781, 13841, 14591, 14621, 14821, 14831, 14891, 15731, 15791, 18731, 19421, 19531, 19541, 19751, 19841, 123731, 123821, 124951, 124981, 125641, 125651, 125791, 125821, 125941, 126761, 126851

Offset: 1

Author

Omar E. Pol, Dec 21 2010

Keywords

• nonn
• base
• fini
• full

Comments

Intersection of emirps A006567 and mountain numbers A134941.

The smallest mountain emirp 1231 and other terms of this sequence was mentioned by Loungrides in Prime Curios! (see link).

Question: How many are there?

There are 602 such terms. - Michael S. Branicky, Dec 31 2021

Examples

			Illustration of a(11) = 13751 as a mountain emirp:
  . . . . .
  . . . . .
  . . 7 . .
  . . . . .
  . . . 5 .
  . . . . .
  . 3 . . .
  . . . . .
  1 . . . 1
		

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..602 (full sequence)
• G. L. Honaker, Jr. and C. K. Caldwell, Prime Curios! 1231

Crossrefs

Cf. A006567, A134941, A134951, A135417, A173071.

Programs

Python

# uses A134941()
from sympy import isprime
def is_emirp(n):
    if not isprime(n): return False
    revn = int(str(n)[::-1])
    return n != revn and isprime(revn)
print([k for k in A134941() if is_emirp(k)]) # Michael S. Branicky, Dec 31 2021

Formula

A006567 INTERSECT A134941.

Extensions

More terms from Nathaniel Johnston, Dec 29 2010

Terms a(31) and beyond from Michael S. Branicky, Dec 31 2021