[$10/$10*@\-$*\]m:             {modulo squared and division}
[$m;![$9>][m;!@@+\]#$*+]s:     {sum of squares}
[$0[1ø1>][1ø3+ø3ø=|\1-\]#\%]f: {look for duplicates}

{check happy number}
[
  $1[f;!~2ø1=~&][1+\s;!@]#     {loop over sequence until 1 or duplicate}
  1ø1=                         {return value}
  \[$0=~][@%1-]#%              {drop sequence and counter}
]h:

0 1
"Happy numbers:"
[1ø8=~][h;![" "$.\1+\]?1+]#
%%