OFFSET
1,1
COMMENTS
The sigma() in the definition is the sum-of-divisors function A000203.
If m is negative, the definition uses the convention sigma(m) = sigma(-m).
EXAMPLE
a(4) = 19 because sigma(19 + 4) = sigma(23) = 1 + 23 = 24 and sigma(19 - 4) = sigma(15) = 1 + 3 + 5 + 15 = 24 and there is no k < 19 for which sigma(k + 4) = sigma(k - 4).
a(26) = 5 because sigma(5 + 26) = sigma(31) = 1 + 31 = 32 and sigma(5 - 26) = sigma(-21) = sigma(21) = 1 + 3 + 7 + 21 = 32.
MATHEMATICA
Table[Min[Select[Range[500], DivisorSigma[1, # - n] == DivisorSigma[1, # + n] &]], {n, 50}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jayanta Basu, Mar 24 2013
STATUS
approved