Help with the question if the following is a linear transformation or not
$begingroup$
I am new to Linear Algebra, and am asked to check if the following transformation is linear. In principle, I know the criteria to check for: T(v+v')=T(v)+T(v') and c(Tv)=T(cv), where v is a vector, c is any scalar, and T is the transformation. However, when applying these to the following transformation, I get completely lost because of the complexity of the actual transformation. I am thinking there must be a shortcut I am not seeing. Any help would be very much appreciated.
$T:M^C_{nxn}to M^C_{nxn}$ defined by $T(X)=2X+(1+3i)bar X^t$. $M^C_{nxn}$ is a linear space over C.
There is an additional comment stating that we should refer to the general element of matrix $X$ as $x_{ij}$, while referring to the general element of matrix $bar X$ as $bar x_{ij}$
Thank you!
linear-algebra linear-transformations
$endgroup$
add a comment |
$begingroup$
I am new to Linear Algebra, and am asked to check if the following transformation is linear. In principle, I know the criteria to check for: T(v+v')=T(v)+T(v') and c(Tv)=T(cv), where v is a vector, c is any scalar, and T is the transformation. However, when applying these to the following transformation, I get completely lost because of the complexity of the actual transformation. I am thinking there must be a shortcut I am not seeing. Any help would be very much appreciated.
$T:M^C_{nxn}to M^C_{nxn}$ defined by $T(X)=2X+(1+3i)bar X^t$. $M^C_{nxn}$ is a linear space over C.
There is an additional comment stating that we should refer to the general element of matrix $X$ as $x_{ij}$, while referring to the general element of matrix $bar X$ as $bar x_{ij}$
Thank you!
linear-algebra linear-transformations
$endgroup$
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16
add a comment |
$begingroup$
I am new to Linear Algebra, and am asked to check if the following transformation is linear. In principle, I know the criteria to check for: T(v+v')=T(v)+T(v') and c(Tv)=T(cv), where v is a vector, c is any scalar, and T is the transformation. However, when applying these to the following transformation, I get completely lost because of the complexity of the actual transformation. I am thinking there must be a shortcut I am not seeing. Any help would be very much appreciated.
$T:M^C_{nxn}to M^C_{nxn}$ defined by $T(X)=2X+(1+3i)bar X^t$. $M^C_{nxn}$ is a linear space over C.
There is an additional comment stating that we should refer to the general element of matrix $X$ as $x_{ij}$, while referring to the general element of matrix $bar X$ as $bar x_{ij}$
Thank you!
linear-algebra linear-transformations
$endgroup$
I am new to Linear Algebra, and am asked to check if the following transformation is linear. In principle, I know the criteria to check for: T(v+v')=T(v)+T(v') and c(Tv)=T(cv), where v is a vector, c is any scalar, and T is the transformation. However, when applying these to the following transformation, I get completely lost because of the complexity of the actual transformation. I am thinking there must be a shortcut I am not seeing. Any help would be very much appreciated.
$T:M^C_{nxn}to M^C_{nxn}$ defined by $T(X)=2X+(1+3i)bar X^t$. $M^C_{nxn}$ is a linear space over C.
There is an additional comment stating that we should refer to the general element of matrix $X$ as $x_{ij}$, while referring to the general element of matrix $bar X$ as $bar x_{ij}$
Thank you!
linear-algebra linear-transformations
linear-algebra linear-transformations
asked Dec 31 '18 at 15:04
daltadalta
11311
11311
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16
add a comment |
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Hint:
Apply the rules mechanically:
$$2(X+X')+(1+3i)overline{(X+X')}^t
\=2X+2X'+(1+3i)(bar X+bar X')^t
\=2X+2X'+(1+3i)(bar X^t+bar X'^t)
\=2X+2X'+(1+3i)bar X+(1+3i)bar X'
\=(2X+(1+3i)bar X)+(2X'+(1+3i)bar X').$$
There is nothing really complicated.
You may also just state that
- averaging is a linear operation,
- transposition is a linear operation,
- scalar multiplicationis a linear operation,
- addition is a linear operation.
Proving each of theses statements separately is at your reach.
Update:
I didn't see that the overline denotes conjugation. Then
- complex conjugation is not is a linear operation.
Because $$overline{aX}=overline aoverline Xne aoverline X.$$
$endgroup$
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
|
show 4 more comments
$begingroup$
The transformation clearly satisfies $T(X+Y) = T(X)+T(Y)$ right? Also if $alpha$ is real then $T(alpha X) = alpha T(X)$ is trivial. However the final property we need for linearity is that $T(iX) = iT(X)$ for then we can conclude that $T((alpha+ibeta)X) = alpha T(X)+ibeta T(X)$ for real $alpha,beta$. We therefore consider
$$2iX+i(1+3i)bar{X}^t=i(2X+(1+3i)bar{X}^t) = iT(X) =^? T(iX) = 2iX-i(1+3i)bar{X}^t$$
If we consider the identity matrix $I$ then is it true that
$$2iI+i(1+3i)I =^? 2iI-i(1+3i)I$$
$endgroup$
add a comment |
Your Answer
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3057788%2fhelp-with-the-question-if-the-following-is-a-linear-transformation-or-not%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint:
Apply the rules mechanically:
$$2(X+X')+(1+3i)overline{(X+X')}^t
\=2X+2X'+(1+3i)(bar X+bar X')^t
\=2X+2X'+(1+3i)(bar X^t+bar X'^t)
\=2X+2X'+(1+3i)bar X+(1+3i)bar X'
\=(2X+(1+3i)bar X)+(2X'+(1+3i)bar X').$$
There is nothing really complicated.
You may also just state that
- averaging is a linear operation,
- transposition is a linear operation,
- scalar multiplicationis a linear operation,
- addition is a linear operation.
Proving each of theses statements separately is at your reach.
Update:
I didn't see that the overline denotes conjugation. Then
- complex conjugation is not is a linear operation.
Because $$overline{aX}=overline aoverline Xne aoverline X.$$
$endgroup$
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
|
show 4 more comments
$begingroup$
Hint:
Apply the rules mechanically:
$$2(X+X')+(1+3i)overline{(X+X')}^t
\=2X+2X'+(1+3i)(bar X+bar X')^t
\=2X+2X'+(1+3i)(bar X^t+bar X'^t)
\=2X+2X'+(1+3i)bar X+(1+3i)bar X'
\=(2X+(1+3i)bar X)+(2X'+(1+3i)bar X').$$
There is nothing really complicated.
You may also just state that
- averaging is a linear operation,
- transposition is a linear operation,
- scalar multiplicationis a linear operation,
- addition is a linear operation.
Proving each of theses statements separately is at your reach.
Update:
I didn't see that the overline denotes conjugation. Then
- complex conjugation is not is a linear operation.
Because $$overline{aX}=overline aoverline Xne aoverline X.$$
$endgroup$
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
|
show 4 more comments
$begingroup$
Hint:
Apply the rules mechanically:
$$2(X+X')+(1+3i)overline{(X+X')}^t
\=2X+2X'+(1+3i)(bar X+bar X')^t
\=2X+2X'+(1+3i)(bar X^t+bar X'^t)
\=2X+2X'+(1+3i)bar X+(1+3i)bar X'
\=(2X+(1+3i)bar X)+(2X'+(1+3i)bar X').$$
There is nothing really complicated.
You may also just state that
- averaging is a linear operation,
- transposition is a linear operation,
- scalar multiplicationis a linear operation,
- addition is a linear operation.
Proving each of theses statements separately is at your reach.
Update:
I didn't see that the overline denotes conjugation. Then
- complex conjugation is not is a linear operation.
Because $$overline{aX}=overline aoverline Xne aoverline X.$$
$endgroup$
Hint:
Apply the rules mechanically:
$$2(X+X')+(1+3i)overline{(X+X')}^t
\=2X+2X'+(1+3i)(bar X+bar X')^t
\=2X+2X'+(1+3i)(bar X^t+bar X'^t)
\=2X+2X'+(1+3i)bar X+(1+3i)bar X'
\=(2X+(1+3i)bar X)+(2X'+(1+3i)bar X').$$
There is nothing really complicated.
You may also just state that
- averaging is a linear operation,
- transposition is a linear operation,
- scalar multiplicationis a linear operation,
- addition is a linear operation.
Proving each of theses statements separately is at your reach.
Update:
I didn't see that the overline denotes conjugation. Then
- complex conjugation is not is a linear operation.
Because $$overline{aX}=overline aoverline Xne aoverline X.$$
edited Dec 31 '18 at 15:25
answered Dec 31 '18 at 15:17
Yves DaoustYves Daoust
134k676232
134k676232
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
|
show 4 more comments
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
But its not linear right?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:20
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
@OlofRubin: why not ? I just explained that it is.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:21
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
$T(iX)neq iT(X)$ in general?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
@OlofRubin: how so ? Please substantitate.
$endgroup$
– Yves Daoust
Dec 31 '18 at 15:22
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
$begingroup$
$T(iI) = 2iI-i(1+3i)I = iI+3I$ while $iT(I) = 2iI+i(1+3i)I = 3iI-3I$?
$endgroup$
– Olof Rubin
Dec 31 '18 at 15:23
|
show 4 more comments
$begingroup$
The transformation clearly satisfies $T(X+Y) = T(X)+T(Y)$ right? Also if $alpha$ is real then $T(alpha X) = alpha T(X)$ is trivial. However the final property we need for linearity is that $T(iX) = iT(X)$ for then we can conclude that $T((alpha+ibeta)X) = alpha T(X)+ibeta T(X)$ for real $alpha,beta$. We therefore consider
$$2iX+i(1+3i)bar{X}^t=i(2X+(1+3i)bar{X}^t) = iT(X) =^? T(iX) = 2iX-i(1+3i)bar{X}^t$$
If we consider the identity matrix $I$ then is it true that
$$2iI+i(1+3i)I =^? 2iI-i(1+3i)I$$
$endgroup$
add a comment |
$begingroup$
The transformation clearly satisfies $T(X+Y) = T(X)+T(Y)$ right? Also if $alpha$ is real then $T(alpha X) = alpha T(X)$ is trivial. However the final property we need for linearity is that $T(iX) = iT(X)$ for then we can conclude that $T((alpha+ibeta)X) = alpha T(X)+ibeta T(X)$ for real $alpha,beta$. We therefore consider
$$2iX+i(1+3i)bar{X}^t=i(2X+(1+3i)bar{X}^t) = iT(X) =^? T(iX) = 2iX-i(1+3i)bar{X}^t$$
If we consider the identity matrix $I$ then is it true that
$$2iI+i(1+3i)I =^? 2iI-i(1+3i)I$$
$endgroup$
add a comment |
$begingroup$
The transformation clearly satisfies $T(X+Y) = T(X)+T(Y)$ right? Also if $alpha$ is real then $T(alpha X) = alpha T(X)$ is trivial. However the final property we need for linearity is that $T(iX) = iT(X)$ for then we can conclude that $T((alpha+ibeta)X) = alpha T(X)+ibeta T(X)$ for real $alpha,beta$. We therefore consider
$$2iX+i(1+3i)bar{X}^t=i(2X+(1+3i)bar{X}^t) = iT(X) =^? T(iX) = 2iX-i(1+3i)bar{X}^t$$
If we consider the identity matrix $I$ then is it true that
$$2iI+i(1+3i)I =^? 2iI-i(1+3i)I$$
$endgroup$
The transformation clearly satisfies $T(X+Y) = T(X)+T(Y)$ right? Also if $alpha$ is real then $T(alpha X) = alpha T(X)$ is trivial. However the final property we need for linearity is that $T(iX) = iT(X)$ for then we can conclude that $T((alpha+ibeta)X) = alpha T(X)+ibeta T(X)$ for real $alpha,beta$. We therefore consider
$$2iX+i(1+3i)bar{X}^t=i(2X+(1+3i)bar{X}^t) = iT(X) =^? T(iX) = 2iX-i(1+3i)bar{X}^t$$
If we consider the identity matrix $I$ then is it true that
$$2iI+i(1+3i)I =^? 2iI-i(1+3i)I$$
answered Dec 31 '18 at 15:14
Olof RubinOlof Rubin
883317
883317
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3057788%2fhelp-with-the-question-if-the-following-is-a-linear-transformation-or-not%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
$begingroup$
Try writing the statement and proof for $n=2$. With $2 times 2$ matrices you won't need any indices for the "general element". Then work on the general case. You can edit the question to show us how far you get before you're stuck.
$endgroup$
– Ethan Bolker
Dec 31 '18 at 15:08
$begingroup$
I fail to see why the subexpression $S: X mapsto bar{X}$ should be linear. $S(lambda X) = bar{lambda} S(X)$, so it isn't linear. Anyway, for OP it's probably easier to prove linearity of $Xmapsto 2X$, $S$, $Xmapsto X^t$ and $Xmapsto (1+3i)X$ separately and then apply "addition of linear functions is linear".
$endgroup$
– ComFreek
Dec 31 '18 at 15:16