login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A048889 Composite numbers not ending in zero that yield a prime when turned upside down. 2
68, 116, 118, 161, 166, 169, 188, 608, 616, 1006, 1018, 1066, 1081, 1096, 1106, 1108, 1118, 1169, 1186, 1606, 1618, 1808, 1816, 1898, 1916, 1918, 1969, 1981, 1988, 1996, 6001, 6008, 6016, 6098, 6188, 6191, 6196, 6616, 6668, 6698, 6808, 6809, 6881, 6896 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel and Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 (a(1) to a(1048) from Reinhard Zumkeller)

EXAMPLE

68 is not prime, yet when turned upside down gives 89 which is prime.

MAPLE

N:= 1000: # to get a(1) to a(N)

count:= 0:

for q from 1 while count < N do

  if q mod 5 <> 0 then

    L:= convert(q, base, 5);

    m:= nops(L);

    Lx:= subs(2=6, 3=8, 4=9, L);

    x:= add(Lx[i]*10^(i-1), i=1..m);

    if isprime(x) then next fi;

    Ly:= subs(2=9, 3=8, 4=6, L);

    y:= add(Ly[-i]*10^(i-1), i=1..m);

    if isprime(y) then

      count:= count+1;

      A[count]:= x;

    fi

fi

od:

seq(A[i], i=1..count); # Robert Israel, Jul 11 2016

PROG

(Haskell)

import Data.List (intersect)

import Numeric (readInt)

import Data.Char (digitToInt)

a048889 n = a048889_list !! (n-1)

a048889_list = filter f a002808_list where

   f n = n `mod` 10 > 0 &&

         null ("23547" `intersect` show n)  &&

         (a010051 (fst $ head $ readInt 10 (const True) ud $ ns) == 1)

       where ns = reverse $ show n

             ud '6' = 9

             ud '9' = 6

             ud z = digitToInt z

-- Reinhard Zumkeller, Aug 11 2011

CROSSREFS

Cf. A002808, A010051.

Sequence in context: A039540 A235284 A250765 * A111379 A044191 A044572

Adjacent sequences:  A048886 A048887 A048888 * A048890 A048891 A048892

KEYWORD

base,easy,nonn,nice

AUTHOR

G. L. Honaker, Jr.

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified April 25 00:32 EDT 2017. Contains 285346 sequences.