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A210625
Least semiprime dividing digit reversal of n, or 0 if no such factor.
1
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
1,4
COMMENTS
Roughly the analog of A209190 (least prime factor of reversal of digits), but with semiprimes (A001358) instead of primes (A000040).
LINKS
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
KEYWORD
nonn,base,look,easy
AUTHOR
Jonathan Vos Post, Mar 24 2012
STATUS
approved