sum of squares of digits of number n
$begingroup$
I have to found a limit of $B/n$, where $B$ is a sum of squares of digits of number $n$. I was thinking about using Cesaro-Stolz theorem, but I'm stuck with the $B$.
analysis elementary-number-theory limits-without-lhopital
$endgroup$
add a comment |
$begingroup$
I have to found a limit of $B/n$, where $B$ is a sum of squares of digits of number $n$. I was thinking about using Cesaro-Stolz theorem, but I'm stuck with the $B$.
analysis elementary-number-theory limits-without-lhopital
$endgroup$
1
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
1
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49
add a comment |
$begingroup$
I have to found a limit of $B/n$, where $B$ is a sum of squares of digits of number $n$. I was thinking about using Cesaro-Stolz theorem, but I'm stuck with the $B$.
analysis elementary-number-theory limits-without-lhopital
$endgroup$
I have to found a limit of $B/n$, where $B$ is a sum of squares of digits of number $n$. I was thinking about using Cesaro-Stolz theorem, but I'm stuck with the $B$.
analysis elementary-number-theory limits-without-lhopital
analysis elementary-number-theory limits-without-lhopital
edited Jan 10 at 20:03
David G. Stork
11k41432
11k41432
asked Jan 10 at 19:44
MilOMilO
83
83
1
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
1
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49
add a comment |
1
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
1
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49
1
1
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
1
1
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Let $$n=overline{a_k a_{k-1}cdots a_0}=10^ka_k+cdots +a_0$$when $a_kne 0$. Therefore$${B_nover n}={a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le {81(k+1)over 10^k}$$and we obtain$$0le lim_{nto infty}{B_nover n}=lim_{kto infty}{a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le lim_{kto infty}{81(k+1)over 10^k}=0$$therefore$$lim_{nto infty}{B_nover n}=0$$
$endgroup$
add a comment |
$begingroup$
Consider a number $n$
in base $b$
with $d$ digits,
the high order digit
being non-zero.
Then
$b^{d-1} le n < b^d$
(so that
$d-1 le log_b(n) < d$)
and
$B_m(n)$,
the sum of the $m$-th powers of the digits of $n$
in base $b$,
satisfies
$begin{array}\
B_m(n)
&le d(b-1)^m\
< (log_b(n)+1)(b-1)^m\
text{so}\
dfrac{B_m(n)}{n}
< dfrac{(log_b(n)+1)(b-1)^m}{n}\
&= dfrac{(ln(n)+ln(b))(b-1)^m}{nln(b)}\
< dfrac{(2sqrt{n}+ln(b))(b-1)^m}{nln(b)}
qquadtext{since }ln(n) < 2sqrt{n}\
< dfrac{3sqrt{n}(b-1)^m}{nln(b)}
qquadtext{for } n > (ln(b))^2\
< dfrac{3(b-1)^m}{sqrt{n}ln(b)}\
< epsilon
qquadtext{for } n > left(dfrac{3(b-1)^m}{epsilonln(b)}right)^2\
&to 0\
end{array}
$
This requires
$n > dfrac{c(b, m)}{epsilon^2}
$
where $c(b, m)$
is an expression that depends on
$b$ and $m$.
For any $a > 0$,
this can be improved to
$n > dfrac{c(b, m, a)}{epsilon^{1+a}}
$
where $c(b, m)$
is a constant that depends on
$b$, $m$, and $a$.
I don't know if
this can be improved to
$n > dfrac{c_0(b, m)}{epsilon}
$
for some $c_0(b, m)$.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Let $$n=overline{a_k a_{k-1}cdots a_0}=10^ka_k+cdots +a_0$$when $a_kne 0$. Therefore$${B_nover n}={a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le {81(k+1)over 10^k}$$and we obtain$$0le lim_{nto infty}{B_nover n}=lim_{kto infty}{a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le lim_{kto infty}{81(k+1)over 10^k}=0$$therefore$$lim_{nto infty}{B_nover n}=0$$
$endgroup$
add a comment |
$begingroup$
Let $$n=overline{a_k a_{k-1}cdots a_0}=10^ka_k+cdots +a_0$$when $a_kne 0$. Therefore$${B_nover n}={a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le {81(k+1)over 10^k}$$and we obtain$$0le lim_{nto infty}{B_nover n}=lim_{kto infty}{a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le lim_{kto infty}{81(k+1)over 10^k}=0$$therefore$$lim_{nto infty}{B_nover n}=0$$
$endgroup$
add a comment |
$begingroup$
Let $$n=overline{a_k a_{k-1}cdots a_0}=10^ka_k+cdots +a_0$$when $a_kne 0$. Therefore$${B_nover n}={a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le {81(k+1)over 10^k}$$and we obtain$$0le lim_{nto infty}{B_nover n}=lim_{kto infty}{a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le lim_{kto infty}{81(k+1)over 10^k}=0$$therefore$$lim_{nto infty}{B_nover n}=0$$
$endgroup$
Let $$n=overline{a_k a_{k-1}cdots a_0}=10^ka_k+cdots +a_0$$when $a_kne 0$. Therefore$${B_nover n}={a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le {81(k+1)over 10^k}$$and we obtain$$0le lim_{nto infty}{B_nover n}=lim_{kto infty}{a_0^2+cdots +a_k^2over 10^ka_k+cdots +a_0}le lim_{kto infty}{81(k+1)over 10^k}=0$$therefore$$lim_{nto infty}{B_nover n}=0$$
answered Jan 10 at 19:56
Mostafa AyazMostafa Ayaz
15.7k3939
15.7k3939
add a comment |
add a comment |
$begingroup$
Consider a number $n$
in base $b$
with $d$ digits,
the high order digit
being non-zero.
Then
$b^{d-1} le n < b^d$
(so that
$d-1 le log_b(n) < d$)
and
$B_m(n)$,
the sum of the $m$-th powers of the digits of $n$
in base $b$,
satisfies
$begin{array}\
B_m(n)
&le d(b-1)^m\
< (log_b(n)+1)(b-1)^m\
text{so}\
dfrac{B_m(n)}{n}
< dfrac{(log_b(n)+1)(b-1)^m}{n}\
&= dfrac{(ln(n)+ln(b))(b-1)^m}{nln(b)}\
< dfrac{(2sqrt{n}+ln(b))(b-1)^m}{nln(b)}
qquadtext{since }ln(n) < 2sqrt{n}\
< dfrac{3sqrt{n}(b-1)^m}{nln(b)}
qquadtext{for } n > (ln(b))^2\
< dfrac{3(b-1)^m}{sqrt{n}ln(b)}\
< epsilon
qquadtext{for } n > left(dfrac{3(b-1)^m}{epsilonln(b)}right)^2\
&to 0\
end{array}
$
This requires
$n > dfrac{c(b, m)}{epsilon^2}
$
where $c(b, m)$
is an expression that depends on
$b$ and $m$.
For any $a > 0$,
this can be improved to
$n > dfrac{c(b, m, a)}{epsilon^{1+a}}
$
where $c(b, m)$
is a constant that depends on
$b$, $m$, and $a$.
I don't know if
this can be improved to
$n > dfrac{c_0(b, m)}{epsilon}
$
for some $c_0(b, m)$.
$endgroup$
add a comment |
$begingroup$
Consider a number $n$
in base $b$
with $d$ digits,
the high order digit
being non-zero.
Then
$b^{d-1} le n < b^d$
(so that
$d-1 le log_b(n) < d$)
and
$B_m(n)$,
the sum of the $m$-th powers of the digits of $n$
in base $b$,
satisfies
$begin{array}\
B_m(n)
&le d(b-1)^m\
< (log_b(n)+1)(b-1)^m\
text{so}\
dfrac{B_m(n)}{n}
< dfrac{(log_b(n)+1)(b-1)^m}{n}\
&= dfrac{(ln(n)+ln(b))(b-1)^m}{nln(b)}\
< dfrac{(2sqrt{n}+ln(b))(b-1)^m}{nln(b)}
qquadtext{since }ln(n) < 2sqrt{n}\
< dfrac{3sqrt{n}(b-1)^m}{nln(b)}
qquadtext{for } n > (ln(b))^2\
< dfrac{3(b-1)^m}{sqrt{n}ln(b)}\
< epsilon
qquadtext{for } n > left(dfrac{3(b-1)^m}{epsilonln(b)}right)^2\
&to 0\
end{array}
$
This requires
$n > dfrac{c(b, m)}{epsilon^2}
$
where $c(b, m)$
is an expression that depends on
$b$ and $m$.
For any $a > 0$,
this can be improved to
$n > dfrac{c(b, m, a)}{epsilon^{1+a}}
$
where $c(b, m)$
is a constant that depends on
$b$, $m$, and $a$.
I don't know if
this can be improved to
$n > dfrac{c_0(b, m)}{epsilon}
$
for some $c_0(b, m)$.
$endgroup$
add a comment |
$begingroup$
Consider a number $n$
in base $b$
with $d$ digits,
the high order digit
being non-zero.
Then
$b^{d-1} le n < b^d$
(so that
$d-1 le log_b(n) < d$)
and
$B_m(n)$,
the sum of the $m$-th powers of the digits of $n$
in base $b$,
satisfies
$begin{array}\
B_m(n)
&le d(b-1)^m\
< (log_b(n)+1)(b-1)^m\
text{so}\
dfrac{B_m(n)}{n}
< dfrac{(log_b(n)+1)(b-1)^m}{n}\
&= dfrac{(ln(n)+ln(b))(b-1)^m}{nln(b)}\
< dfrac{(2sqrt{n}+ln(b))(b-1)^m}{nln(b)}
qquadtext{since }ln(n) < 2sqrt{n}\
< dfrac{3sqrt{n}(b-1)^m}{nln(b)}
qquadtext{for } n > (ln(b))^2\
< dfrac{3(b-1)^m}{sqrt{n}ln(b)}\
< epsilon
qquadtext{for } n > left(dfrac{3(b-1)^m}{epsilonln(b)}right)^2\
&to 0\
end{array}
$
This requires
$n > dfrac{c(b, m)}{epsilon^2}
$
where $c(b, m)$
is an expression that depends on
$b$ and $m$.
For any $a > 0$,
this can be improved to
$n > dfrac{c(b, m, a)}{epsilon^{1+a}}
$
where $c(b, m)$
is a constant that depends on
$b$, $m$, and $a$.
I don't know if
this can be improved to
$n > dfrac{c_0(b, m)}{epsilon}
$
for some $c_0(b, m)$.
$endgroup$
Consider a number $n$
in base $b$
with $d$ digits,
the high order digit
being non-zero.
Then
$b^{d-1} le n < b^d$
(so that
$d-1 le log_b(n) < d$)
and
$B_m(n)$,
the sum of the $m$-th powers of the digits of $n$
in base $b$,
satisfies
$begin{array}\
B_m(n)
&le d(b-1)^m\
< (log_b(n)+1)(b-1)^m\
text{so}\
dfrac{B_m(n)}{n}
< dfrac{(log_b(n)+1)(b-1)^m}{n}\
&= dfrac{(ln(n)+ln(b))(b-1)^m}{nln(b)}\
< dfrac{(2sqrt{n}+ln(b))(b-1)^m}{nln(b)}
qquadtext{since }ln(n) < 2sqrt{n}\
< dfrac{3sqrt{n}(b-1)^m}{nln(b)}
qquadtext{for } n > (ln(b))^2\
< dfrac{3(b-1)^m}{sqrt{n}ln(b)}\
< epsilon
qquadtext{for } n > left(dfrac{3(b-1)^m}{epsilonln(b)}right)^2\
&to 0\
end{array}
$
This requires
$n > dfrac{c(b, m)}{epsilon^2}
$
where $c(b, m)$
is an expression that depends on
$b$ and $m$.
For any $a > 0$,
this can be improved to
$n > dfrac{c(b, m, a)}{epsilon^{1+a}}
$
where $c(b, m)$
is a constant that depends on
$b$, $m$, and $a$.
I don't know if
this can be improved to
$n > dfrac{c_0(b, m)}{epsilon}
$
for some $c_0(b, m)$.
answered Jan 10 at 21:52
marty cohenmarty cohen
74.2k549128
74.2k549128
add a comment |
add a comment |
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1
$begingroup$
Hint: $Bleq9^2log_{10}(n)$, more accurately $Bleq9^2(lfloorlog_{10}(n)rfloor+1)$
$endgroup$
– SmileyCraft
Jan 10 at 19:45
$begingroup$
Wow, I didn't know that fact. Thank you!
$endgroup$
– MilO
Jan 10 at 19:46
1
$begingroup$
You should try to justify it though
$endgroup$
– SmileyCraft
Jan 10 at 19:47
$begingroup$
Hint: suppose $n = underbrace{999cdots 999}_n$.
$endgroup$
– David G. Stork
Jan 10 at 19:49