be a prime number. Find all positive integers 
such that:![\[a^p+b^p=p^c.\]](https://latex.artofproblemsolving.com/3/e/6/3e6307d51ca1995a56718cb724a0a09c40174ec4.png)
[/et_pb_text][/et_pb_column][/et_pb_row][et_pb_row _builder_version="3.25"][et_pb_column type="4_4" _builder_version="3.25" custom_padding="|||" custom_padding__hover="|||"][et_pb_accordion open_toggle_text_color="#0c71c3" _builder_version="3.22.4" toggle_font="||||||||" body_font="Raleway||||||||" text_orientation="center" custom_margin="10px||10px"][et_pb_accordion_item title="Source of the problem" _builder_version="4.0" open="on"]France IMO TST 2012, Problem 3[/et_pb_accordion_item][et_pb_accordion_item title="Topic" open="off" _builder_version="4.0"]Number Theory[/et_pb_accordion_item][et_pb_accordion_item title="Difficulty Level" _builder_version="4.0" open="off"]7/10 [/et_pb_accordion_item][et_pb_accordion_item title="Suggested Book" _builder_version="4.0" open="off"]Challenges and Thrills of Pre College Mathematics[/et_pb_accordion_item][/et_pb_accordion][et_pb_text _builder_version="3.27.4" text_font="Raleway|300|||||||" text_text_color="#ffffff" header_font="Raleway|300|||||||" header_text_color="#e2e2e2" background_color="#0c71c3" custom_margin="48px||48px" custom_padding="20px|20px|20px|20px" border_radii="on|5px|5px|5px|5px" box_shadow_style="preset3"]
and 
. [/et_pb_tab][et_pb_tab title="Hint 3" _builder_version="4.0" hover_enabled="0"]Case 2: p is an odd primeThis now requires the idea of Lifting the Exponents. Please read here if you don't know it.It is an advanced technique to deal with Diophantine Equations.Let's check that the conditions of the LTE are satisfying here.p is an odd prime. gcd(a,b) = 1.p doesn't divide a or b as we are looking for fundamental solutions.\( a^p = a mod p; b^p = b mod p \).Hence, \( a^p + b^p = a + b mod p \).So, p | a+b, and p don't divide a or b. Hence, we can apply LTE.
[/et_pb_tab][et_pb_tab title="Hint 4" _builder_version="4.0" hover_enabled="0"]Using the LTE idea, we get 
Then since 
, we have that 
, so 
Now, you see this can't happen for large p, as the LHS is exploding too fast like exponential as p increases and RHS is linear in p.So, we will apply inequality to prove this and find a bound for p for which it works and search in that bound.Note that 
or 
if 
.Therefore, we must have that 
(or vice versa). But clearly 
, so no solution.QED
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In 2026, the following Cheenta students have been successful for Indian Statistical Institute's M.Stat Entrance. They ranked within the first 50 in the entire country in these entrances. I.S.I. M.Stat Entrance
In 2026, the following Cheenta students have been successful for Indian Statistical Institute's B.Stat Entrance and Chennai Mathematical Institute's B.Sc. Math Entrance. They ranked within the first 200 in the entire country in these entrances. Most of these students attended the problem solving workshops regularly, which happen 5 days every week. CMI B.Sc. Math Entrance […]
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