8128 Small list of interesting 4-digit numbers:   1061    4-digit prime numbers (all) 1105    various sums of two squares 2520    1st highly composite number 4225    various sums of two squares 5525    various sums of two squares 5777    first of two 'Stern numbers' 5993    second of the Stern numbers 6174    Kaprekar's constant 4-digit 8125    various sums of two squares 8128   ⇐ only 4-digit perfect number collected by artmetic   monoalgorithmic digital art horn_torus  real world ↔ complex numbers   ⌂©