121920

Appears in sequences

• Numbers k such that gcd(sigma(k),k) = k/5.at n=17A067237
• Numbers k such that sigma(k) divides k^2.at n=35A090777
• Numbers k for which sigma(k)/k - 1/5 is an integer.at n=5A218406
• G.f.: Sum_{n>=1} x^n * (1+x)^n / (1-x^n).at n=24A227635
• Numbers n such that n = concatenate(a, b) and sigma(a) + sigma(b) = phi(n).at n=12A249065
• Number of (n+2) X (2+2) 0..3 arrays with every 3 X 3 subblock row and column sum equal to 0 2 3 6 or 7 and every 3 X 3 diagonal and antidiagonal sum not equal to 0 2 3 6 or 7.at n=17A252108
• Table where row n lists numbers N equal to the determinant of an n X n circulant having as a row the base n+1 digits of N.at n=50A303262
• Composite numbers whose harmonic mean of their divisors that are larger than 1 is an integer.at n=34A335267
• Numbers whose divisors have a harmonic mean with a denominator 2.at n=30A348411
• Number of (binary) heaps with element set [n] and length n+1.at n=10A373496