
theorem Th1231:
digits(2925,10) = <%5,2,9,2%>
proof
  set d=<%5,2,9,2%>;
  set e=<%5*10|^0,2*10|^1,9*10|^2,2*10|^3%>;
  A1: Sum e = Sum (<%5*10|^0,2*10|^1,9*10|^2%>)+Sum(<%2*10|^3%>) by AFINSQ_2:55
  .= Sum (<%5*10|^0,2*10|^1%>)+Sum(<%9*10|^2%>)+Sum(<%2*10|^3%>) by AFINSQ_2:55
  .= ((5*10|^0)+(2*10|^1))+Sum(<%9*10|^2%>)+Sum(<%2*10|^3%>) by AFINSQ_2:54
  .= ((5*10|^0)+(2*10|^1))+(9*10|^2)+Sum(<%2*10|^3%>) by AFINSQ_2:53
  .= ((5*10|^0)+(2*10|^1))+(9*10|^2)+(2*10|^3) by AFINSQ_2:53
  .= 5*1 + 2*(10|^1) + 9*(10|^2) + 2*(10|^3) by NEWTON:4
  .= 5 + 2*10 + 9*(10|^2) + 2*(10|^3) by NEWTON:5
  .= 25 + 9*(10*10) + 2*(10|^3) by POLYEQ_5:1
  .= 25 + 9*(10*10) + 2*(10*10*10) by POLYEQ_5:2
  .= 2925;
  A2: dom d = 4 by AFINSQ_1:84 .= dom e by AFINSQ_1:84;
  now
    let i be Nat;
    assume i in dom d;
    then i in 4 by AFINSQ_1:84;
    then i in {0,1,2,3} by CARD_1:52;
    then i = 0 or i = 1 or i = 2 or i = 3 by ENUMSET1:def 2;
    hence e.i=(d.i)*10|^i;
  end;
  then A3: value(d,10)=2925 by A1,A2,NUMERAL1:def 1;
  len(d) - 1 = 4-1 by AFINSQ_1:84;
  then A4: d.(len(d)-1) <> 0;
  now
    let i be Nat;
    assume i in dom d;
    then i in 4 by AFINSQ_1:84;
    then i in {0,1,2,3} by CARD_1:52;
    then i = 0 or i = 1 or i = 2 or i = 3 by ENUMSET1:def 2;
    hence 0 <= d.i & d.i < 10;
  end;
  hence thesis by A3,A4,NUMERAL1:def 2;
end;
