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

1,1

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

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

Formula

A006567 INTERSECT A134941.

Example

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

Prog

(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

Crossrefs

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

Sequence in context: A250894 A251405 A278018 * A183881 A270540 A206272

Adjacent sequences: A182718 A182719 A182720 * A182722 A182723 A182724

Keyword

nonn,base,fini,full

Author

Omar E. Pol, Dec 21 2010

Extensions

More terms from Nathaniel Johnston, Dec 29 2010

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

Status

approved