OFFSET
1,1
COMMENTS
a(n) is the smallest prime divisor of A056524(n), or smallest prime divisor of the concatenation of a number n and reverse(n).
LINKS
Michel Lagneau, Table of n, a(n) for n = 1..10000
EXAMPLE
a(10) = 7 because the concatenation of 10 and 01 is 1001 = 7*11*13 where 7 is the smallest divisor of 1001.
MAPLE
with(numtheory):for n from 1 to 100 do:x:=convert(n, base, 10):n1:=nops(x): s:=sum('x[i]*10^(n1-i)', 'i'=1..n1):y:=n*10^n1+s:z:=factorset(y):n2:=nops(z):d:=z[1]:printf(`%d, `, d):od:
MATHEMATICA
d[n_]:=IntegerDigits[n]; Table[FactorInteger[FromDigits[Join[x=d[n], Reverse[x]]]][[1, 1]], {n, 1, 100}]
Table[FactorInteger[#][[1, 1]]&/@Select[Range[10^n, 10^(n+1)-1], PalindromeQ], {n, 1, 3, 2}]//Flatten (* Harvey P. Dale, Jul 19 2021 *)
PROG
(Python)
from sympy import primefactors
def a(n): s = str(n); return min(primefactors(int(s + s[::-1])))
print([a(n) for n in range(1, 83)]) # Michael S. Branicky, Nov 02 2021
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Michel Lagneau, Apr 05 2014
STATUS
approved