\\ Alternating sums of decimal digits of n.
t_altsum (n) = {my (v,p,s);
v=digits(n); p=1; s=0;
for (i=1,#v, s+=p*v[i]; p=-p); s}

t_altsumiter (n) = {my (a);
a=t_altsum(n);
if (#digits(a)<2, a, t_altsumiter(a))}

\\ Harmonic number 1+1/2+...+1/n.
t_harmonic (n) = sum(k=1,n,1/k)