A209190 -id:A209190 - cp's OEIS frontend

A210437 Greatest prime factor of reversal of digits of n.

1, 2, 3, 2, 5, 3, 7, 2, 3, 1, 11, 7, 31, 41, 17, 61, 71, 3, 13, 2, 3, 11, 2, 7, 13, 31, 3, 41, 23, 3, 13, 23, 11, 43, 53, 7, 73, 83, 31, 2, 7, 3, 17, 11, 3, 2, 37, 7, 47, 5, 5, 5, 7, 5, 11, 13, 5, 17, 19, 3, 2, 13, 3, 23, 7, 11, 19, 43, 3, 7, 17, 3, 37, 47, 19

Offset: 1

Alois P. Heinz, Mar 21 2012

Alois P. Heinz, Table of n, a(n) for n = 1..12000

Cf. A004086, A006530, A209190.

with(numtheory):
r:= proc(n) option remember; local q;
      `if`(n<10, n, irem(n, 10, 'q')*10^(length(n)-1) +r(q))
    end:
a:= n-> `if`(r(n)=1, 1, max(factorset(r(n))[])):
seq(a(n), n=1..100);

FactorInteger[FromDigits[Reverse[IntegerDigits[#]]]][[-1,1]]&/@Range[80] (* Harvey P. Dale, Sep 14 2014 *)

gpf(n) = if (n==1, 1, vecmax(factor(n)[,1]));
a(n) = gpf(fromdigits(Vecrev(digits(n)))); \\ Michel Marcus, Feb 11 2020

a(n) = A006530(A004086(n)) = gpf(R(n)). - Jonathan Vos Post, Mar 22 2012

A210625 Least semiprime dividing digit reversal of n, or 0 if no such factor.

Original entry on oeis.org

0, 0, 0, 4, 0, 6, 0, 4, 9, 0, 0, 21, 0, 0, 51, 0, 0, 9, 91, 0, 4, 22, 4, 6, 4, 62, 4, 82, 4, 0, 0, 0, 33, 0, 0, 9, 0, 0, 93, 4, 14, 4, 34, 4, 6, 4, 74, 4, 94, 0, 15, 25, 35, 9, 55, 65, 15, 85, 95, 6, 4, 26, 4, 46, 4, 6, 4, 86, 4, 0, 0, 9, 0, 0, 57, 0, 77, 87

Offset: 1

Jonathan Vos Post, Mar 24 2012

Roughly the analog of A209190 (least prime factor of reversal of digits), but with semiprimes (A001358) instead of primes (A000040).

			a(12) = min {k such that k|R(12) and k = p*q for primes p and q (not necessarily distinct)} = min {k, k|21 and k semiprime} = 21 = 3*7.
a(42) = min {k, k|24 and k semiprime} = min {4,6} = 4 = 2*2.

Alois P. Heinz, Table of n, a(n) for n = 1..10000

Cf. A001358, A004086, A004087, A011557, A209190, A210615.

r:= proc(n) option remember; local q;
      `if`(n<10, n, irem(n, 10, 'q') *10^(length(n)-1)+r(q))
    end:
a:= proc(n) local m, k;
      m:= r(n);
      for k from 4 to m do
         if irem(m, k)=0 and not isprime(k) and
            add(i[2], i=ifactors(k)[2])=2 then return k fi
      od; 0
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Mar 26 2012

spd[n_]:=Module[{sps=Select[Divisors[FromDigits[Reverse[ IntegerDigits[n]]]], PrimeOmega[#] == 2&,1]},If[sps=={},0,First[sps]]]; Array[spd,80] (* Harvey P. Dale, Aug 12 2012 *)

a(n) = A210615(R(n)) = A210615(A004086(n)).

a(p) = 0 iff p in (A004087 union A011557). - Alois P. Heinz, Mar 28 2012

cp's OEIS Frontend

A210437 Greatest prime factor of reversal of digits of n.

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