OFFSET
1,1
EXAMPLE
11 is deficient and 12 is abundant.
17 is deficient and 18 is abundant.
MAPLE
with(numtheory): select(n->sigma(n)<2*n and sigma(n+1)>2*(n+1), [$1..400]);
MATHEMATICA
Select[Range@ 240, And[DivisorSigma[1, #] < 2 #, DivisorSigma[1, # + 1] > 2 (# + 1)] &] (* Michael De Vlieger, Jul 01 2018 *)
PROG
(GAP) Filtered([1..400], n->Sigma(n)<2*n and Sigma(n+1)>2*(n+1));
(PARI) isok(n) = (sigma(n) < 2*n) && (sigma(n+1) > 2*(n+1)); \\ Michel Marcus, Jul 02 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Muniru A Asiru, Jul 01 2018
STATUS
approved