Average number is a simplified, restricted polynomial, e.g. Σ(r=a,b) d*B^r ....
.... d represents digit, B represents base, r represents position index.
Or, one can say the average number is a constant, name of standardized
procedure/operation, so we don't need to calculate further, Because most
properties are well known.
Such infinite numbers like the sum of even number (or irrational number if no
name is given) are defined never terminate in 'normal expression'. You cannot
ask it to equal to anything except itself, because it is unnamed.
Adding '∞' cannot express all of them. It just a symbol denoting a kind of
infinity.
For the two expressions:
(1) for(n=0, sum=0;;n+=1) { sum = sum + n }; return sum;
(2) for(n=0, sum=0;;n+=2) { sum = sum + n }; return sum;
This is the good thing using C-like expression to express 'sum'. I have not
really inspected the how part of this. This is the QUESTION of this post for
idea. However, I try to explain what I thought:
(3) for(n=0, sum1=sum2=0;;) {
sum0+=n; ++n;
sum1+=n; ++n;
};
Expression (3) may be considered equal to expression (1) in that sum=sum0+sum1.
(3) is not supposed to 'return', because the process 'never terminate', this
is the definition of infinity. (3) is computed by 'another TM'. As to how we
know 'the value', simply to say, unnamed, 'the value' is constantly changing.
If one really wants a 'value', the whole expression is the value, return the
whole expression. The corresponding reality might be that we take a snapshot
of the 'another TM' to know the sum (and in 'standardized' form).
(3)==(1), so the number expressed by (2) is smaller than that by expression (1), i.e. (2)<(1)
As said, I did not really inspect C-like expression. Such conclusion need
axiomatization.
However, I classify such questions to my 'infinite series' theory, I have a
few theorems about infinite series.
Snippet from
https://sourceforge.net/projects/cscall/files/MisFiles/NumberView-en.txt/download
The article is for ME, added some material for 'communication'. Suggestions of improvement are welcome.
+-----------------+
| Infinite Series |
+-----------------+
Series::= S= Σ(n=0,k) a(n)= a(0)+ a(1)+ a(2) +... +a(k)
a(n) is called the general term, addend, summand. n is referred as the
index. Series S is the sum from the first term a(0) to the last term a(k).
The sum of those first terms (n<k) is called the partial sum.
"a(0)+...+a(k)" is called expanded form.
Infinite Series::= If the series S refers to infinite terms (n=∞), S is called
an infinite series. Note that there are infinite addend. The sum cannot be
completed by enumeration (∞ means unreachable, by definition).
In the concept that number-is-an-expression-of-computation, infinite series is
a number, no such concern of converge/diverge (statement when number converges
is a number, diverges is not, is self-controdictory). The computaion rule of
infinite series is based on the expanded form and concepts mentioned above.
Noteworthy difference is that the interpretation of "..." in the expanded form
is a "fixed/unique" number of terms, i.e. "∞+1≠∞" (not the notion of Cantor's
infinite correspondence).
Arithmetic of expanded form:
Ex1: Let S= Σ(n=0,∞) a^n = 1+a+a^2+...+a^∞)
S= 1+a*(1+a+a^2+...+a^∞)- a*a^∞
<=> S= 1+a*S-a^(∞+1)
<=> S(1-a)=1-a^(∞+1)
<=> S= (1-a^(∞+1))/(1-a)
Ex2: Let S= Σ(n=1,∞) n = 1+2+3+...+n
S= 1+2+3+...+n // (1)
S= n+...+3+2+1 // (2)
2S= n*(n+1) // (1)+(2)
<=> S= n*(n+1)/2
∴ Basically, formula for 'finite' series is applicable to infinite series.
(note that mathematical inducion cannot prove such formulas because by
definition, ∞ is unreachable by counting.)
Rule: Handling of the expanded form of infinite series must list the last
addend. Otherwise, the expanded form is ill-formed (obscure semantics and
information being lost cannot conduct valid deduction).
Ex.1 (the last addend is omitted):
A=1+2+3+4+5+...
=(1+2)+(3+4)+5+...
=3+7+5+... // ill-formed, obscure semantics.
Last addend listed:
A=1+2+3+4+5+...+∞ // well-formed, the exanded form of Σ(n=1,∞) {n}
Ex.2:
S=1+2+4+8+... // ill-formed
<=> S=1+2(1+2+4+8+...)
<=> S=1+2S
<=> S=-1
Last addend listed:
S=1+2+4+8+...+2^∞
<=> S=1+2(1+2+4+...+2^(∞-1))
<=> S=1+2S-2^(∞+1)
<=> S=2^(∞+1)-1 // Lots of similar "magic calculation" deriving the result
// S=-1 can be found in youtube. (the term containing the
// last addend ∞ is ignored)
Ex.3:
"f(n)= Σ(k=0,n) 1/k! => f(∞)=e(The base of natural logarithm)"?
We know for sure ∀n∈ℕ, f(n)∈ℚ. To get the result f(n)=e (f(n)∉ℚ), the only
current option is n=∞. But the issue whether or not f(∞)=e (exact equal by
definition) can only be decided via definition, e.g. e≡f(∞). Otherwise, we
can only say f(∞)≈e. (In considering the definition of the equal sign '=',
other forms of e are likely not mutually replaceable with f(∞))
Ex.4: x= Σ(n=1,∞) 1/n²
A common expression is x= Σ(n=1,∞) 1/n²= π²/6, therefore, π=√(6*x)
The issue here is: Lots of π can be derived from various infinite serieses.
But, according to the definition of '=', the result of mutual substitution
may become inconsistent.
For now, the uncontroversial definition of π is the ratio of the
circumference of a circle to its diameter (no computable definition), it is
more correct to use '≈'.
Therefore, Σ(n=1,∞) 1/n² ≈ π²/6 is what it is.
[Note1] "..." in expression is normally indeterminant, of vague semantic.
"0.999..." is also indeterminant before the "..." is eliminated, the
number "0.999..." represents is uncertain, must be removed to ensure
what the number is.
Ex1: Let x=0.999...
10*x= 9+x // This is the result of x after interpreted, not necessarily
// the result followed from "x=0.999..."
// This equation must be given to define x (eliminate the
// ambiguous "...")
Ex2: Let x=√(2+√(2+√(2+...))). Then, possible interpretation of x are:
x=√(2+x)
x=√(2+√(2+x))
x=√(2+√(2+√(2+x)))
...
Ex3: "0.999..." usual 'repeating decimal' cannot denote a unique number.
Let A= Σ(n=1,∞) 1/2^n = 0.999...
B= Σ(n=1,∞) 9/10^n = 0.999...
Let A=B
<=> 1-1/2^∞= 1-1/10^∞ // converted from the formula of geometric series
<=> 1/2^∞= 1/10^∞
<=> 10^∞= 2^∞
<=> 5^∞=1
<=> false
[Note2] Expanded form is prone to magic tricks, perhaps owing to conceptional
generalization of visual illusion too easy to form. It is an error
because the regrouping of the expanded form hides the fact that the
original way of computation is reformulated.
Ex: S can be the sum of any sequence of natural numbers.
S= Σ(n=1,∞) n= 1+2+3+... =1+1+1+1+...= (1+1)+(1+1+1)+...
= Σ(n=1,∞) n+1 // S is modified
Axiom: Σ(n=0,∞) a(n)= a(0)+ Σ(n=1,∞) a(n)
= a(∞)+ Σ(n=0,∞-1) a(n)
Theorem1: Σ(n=0,∞) f(n) ± Σ(n=0,∞) g(n) = Σ(n=0,∞) f(n)±g(n)
Theorem2: Σ(n=0,∞) c*f(n)= c*(Σ(n=0,∞) f(n))
Proof: Omitted (Can be derived from the expanded form)
Ex1: Σ 2*n =Σ (n+n) =Σ n + Σ n
If Σ 2*n is said the sum of all even numbers, Σ n the sum of all natural
numbers, the notion that the whole is greater than the part is conflicted
by this rule (many paradoxical and current text book arithmetic have the
same issue using Theorem2 like in Ex3).
But, how do we express "the sum of even numbers"? Or Σ(n=0,∞/2) 2*n ?
An idea that using C-language's for loop expression might solve this
problem (or, at least, better than the traditional Σ notation):
for(n=0;;++n) n; or f(n=0;;n+=2) n;
Benefit of such a notation is 1.the symbol '∞' can be omitted 2. the
meaning is more concrete, reducing mathematical imagination of 'Σ'.
Temporary Conclusion: The essence of an infinite series may be a number whose
computation never terminates because of infinite number of non-zero
addends), or could be imagined as a 'running' number (density property
requires the existence of such an 'irrational' number).
---