Last updated: October 4, 2026

This Privacy Policy explains how Euler’s Eyes handles information about people who use:

  • the website www.eulereyes.com; and
  • the Euler Eyes Android app (package com.eulereyes.www.twa), available on Google Play.

The Android app is a Trusted Web Activity: it opens the www.eulereyes.com website inside the app, using the Chrome browser installed on the device. This policy therefore applies equally to the website and the app.

Who we are

Euler Eyes Mathematics (Euler’s Eyes), developer of the website and the app. Contact: hirowat2@gmail.com.

Data we collect

Euler’s Eyes does not require registration, login or payment, and the app does not access contacts, camera, microphone, precise location, files or any other device data.

When you use the website or the app, the following data may be collected automatically:

Data Collected by Purpose
IP address, browser and device type, pages visited, date and time Our server (technical logs) Operation, security and troubleshooting
Cookie and advertising identifiers, pages visited, ad interactions, approximate location (from IP) Google AdSense Showing ads and measuring their performance
Cookie identifiers, pages visited, reading time, approximate region, device type Google Analytics Aggregated audience statistics
Your cookie consent choice Google (consent message) Respecting your choice about cookies and personalized ads

We do not sell personal data and do not use data for any purpose other than those listed above.

Cookies and advertising

The website and the app show ads from Google AdSense. Google and its partners use cookies to serve ads based on previous visits to this and other websites. On your first visit, a consent message lets you accept or decline personalized ads. Without consent, only non-personalized ads are shown.

You can also:

Sharing

The data described above is processed by Google LLC (Google AdSense, Google Analytics and Google Tag Manager) under the Google Privacy Policy. Apart from that, we only share information when required by law or court order.

The website may link to external sites (for example, books on Amazon or the Ko-fi support page). Their own privacy policies apply when you visit them.

Children

The content is educational and may be read by students of all ages, but Euler’s Eyes does not knowingly collect personal data from children and does not ask for any identifying information. Parents or guardians who believe a child’s data has been collected can request deletion at the email above.

Data retention and deletion

  • Server technical logs: kept only as long as needed for security and troubleshooting, and discarded periodically.
  • Google Analytics data: deleted automatically at the end of the configured retention period (at most 14 months).
  • Advertising data: retained by Google under its privacy policy.

To request access to, correction of or deletion of your data, email hirowat2@gmail.com with the subject “Privacy”. We reply within 15 days. Since there are no user accounts, you can delete data stored on your device by clearing your browser data or uninstalling the app.

Your rights

Under Brazil’s General Data Protection Law (LGPD, Law 13.709/2018) and similar laws, you may request confirmation of processing, access, correction, anonymization or deletion of your data, information about sharing, and withdrawal of consent, using the email above.

Security

The website and the app use an encrypted connection (HTTPS) on every page.

Changes

This policy may be updated. The date at the top of the page shows the version in effect.

Contact

Questions about this policy: hirowat2@gmail.com.