login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A237913 Smallest number m > 1 (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity). Palindromes are included. 2

%I

%S 2,4,8,88,252,2576,21708,2112,4224,8448,44544,48384,2977792,21989376,

%T 405504,4091904,441606144,405909504,886898688,677707776,4285005824

%N Smallest number m > 1 (not ending in a 0) such that m and the digit reversal of m have n prime factors (counted with multiplicity). Palindromes are included.

%F a(n) = min{A076886(n+1), A237912(n)}

%e 252 is the smallest number such that 252 and its reverse (also 252) have 5 prime factors (2*2*3*3*7). So, a(5) = 252.

%e 2576 is the smallest number such that 2576 and its reverse (6752) have 6 prime factors (2*2*2*2*7*23 and 2*2*2*2*2*211, respectively). So a(6) = 2576.

%o (Python)

%o import sympy

%o from sympy import factorint

%o def rev(x):

%o ..rev = ''

%o ..for i in str(x):

%o ....rev = i + rev

%o ..return int(rev)

%o def RevFact(x):

%o ..n = 2

%o ..while n < 10**8:

%o ....if n % 10 != 0:

%o ......if sum(list(factorint(n).values())) == x:

%o ........if sum(list(factorint(rev(n)).values())) == x:

%o ..........return n

%o ........else:

%o ..........n += 1

%o ......else:

%o ........n += 1

%o ....else:

%o ......n += 1

%o x = 1

%o while x < 100:

%o ..print(RevFact(x))

%o ..x += 1

%Y Cf. A004086, A076886, A237912.

%K nonn,base,more

%O 1,1

%A _Derek Orr_, Feb 15 2014

%E a(17)-a(21) from _Giovanni Resta_, Feb 23 2014

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 2 08:34 EDT 2022. Contains 357191 sequences. (Running on oeis4.)