A210625 - OEIS

A210625

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

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

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OFFSET

1,4

COMMENTS

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

LINKS

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

FORMULA

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

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

EXAMPLE

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.

MAPLE

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

MATHEMATICA

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 *)

CROSSREFS

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

Sequence in context: A103688 A125961 A016681 * A210615 A179312 A076290

Adjacent sequences: A210622 A210623 A210624 * A210626 A210627 A210628

KEYWORD

nonn,base,look,easy

AUTHOR

Jonathan Vos Post, Mar 24 2012

STATUS

approved