A127828 Largest n-digit emirp (A006567) with nonincreasing digits.

97, 991, 9941, 99881, 999983, 9999971, 99999643, 999999761, 9999999511, 99999999977, 999999998863, 9999999998633, 99999999998333, 999999999999877, 9999999999998633, 99999999999987443, 999999999999999331, 9999999999999988633, 99999999999999999631

Offset

2,1

Links

Michael S. Branicky, Table of n, a(n) for n = 2..527

Mathematica

nde[n_]:=Module[{p=NextPrime[10^n, -1]}, While[Max[Differences[ IntegerDigits[ p]]]>0 || CompositeQ[IntegerReverse[p]], p=NextPrime[ p, -1]]; p]; Array[nde, 17, 2] (* Harvey P. Dale, Mar 13 2020 *)

Prog

(Python)
from sympy import isprime
from itertools import count, islice, combinations_with_replacement as mc
def bgen(d):
    nd = ("".join(m) for m in mc("987654321", d))
    yield from filter(isprime, map(int, nd))
def ok(ndp):
    s = str(ndp)
    return len(set(s)) != 1 and isprime(int(s[::-1]))
def agen():
    yield from (next(filter(ok, bgen(d))) for d in count(2))
print(list(islice(agen(), 20))) # Michael S. Branicky, Jun 26 2022

Crossrefs

Cf. A006567, A030540, A114019, A127748, A127827.

Sequence in context: A068852 A093944 A127748 * A186075 A087596 A133836

Adjacent sequences: A127825 A127826 A127827 * A127829 A127830 A127831

Keyword

nonn,base

Author

Ray Chandler, Jan 31 2007

Extensions

a(19) and beyond from Michael S. Branicky, Jun 26 2022

Status

approved