OFFSET
1,1
EXAMPLE
sigma(1909) - 1909 = 107 that is a prime number;
sigma(1910) - 1910 = 1546 = 2*773;
sigma(1911) - 1911 = 1281 = 3*7*61;
sigma(1912) - 1912 = 1688 = 2*2*2*211.
No other number < 1909 has this property and therefore a(4) = 1909.
MAPLE
with(numtheory): P:=proc(q) local a, b, d, i, j, k, ok, n; d:=1;
for k from 1 to q do for n from d to q do ok:=1; for j from 1 to k do
b:=ifactors(sigma(n+j-1)-n-j+1)[2]; if add(b[i][2], i=1..nops(b))<>j then ok:=0; break; fi; od;
if ok=1 then d:=n; print(n); break; fi; od; od; end: P(10^12);
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Paolo P. Lava, Dec 14 2016
EXTENSIONS
a(7)-a(8) from Giovanni Resta, Dec 14 2016
STATUS
approved