OFFSET
1,2
COMMENTS
If d are divisors of n then values of sequence a(n) are the bending moments at point 0 of static forces of sizes tau(d) operating in places d on the cantilever as the nonnegative number axis of length n with support at point 0 by the schema: a(n) = (n * tau(n)) - Sum_{(d<n) | n} (d * tau(d)).
If a(n) = 0 then n must be > 10^7.
Conjecture: a(n) = sigma(n) iff n is a power of 2 (A000079).
Number n = 72 is the smallest number n such that a(n) < n (see A245213).
Number n = 144 is the smallest number n such that a(n) < 0 (see A245214).
LINKS
Jens Kruse Andersen, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
For n = 6 with divisors [1, 2, 3, 6] we have: a(6) = 6 * tau(6) - (3 * tau(3) + 2 * tau(2) + 1 * tau(1)) = 6*4 - (3*2+2*2+1*1) = 13.
PROG
(Magma) [(2*(n*(#[d: d in Divisors(n)]))-(&+[d*#([e: e in Divisors(d)]): d in Divisors(n)])): n in [1..1000]];
(PARI) a(n) = sumdiv(n, d, (-1)^(d<n)*d*numdiv(d)) \\ Jens Kruse Andersen, Aug 13 2014
CROSSREFS
KEYWORD
sign
AUTHOR
Jaroslav Krizek, Jul 23 2014
STATUS
approved