MathLabs

Problem 1

Find all quadruples of positive integers (a,p,m,o)(a, p, m, o) with o≥2o \ge 2 such that a!+p!=mo+26a! + p! = m^o + 26.
Step 2 of 5: Eliminate p=1p=1
p=1: a!=mo+25, a≥5, 5∣m, m odd, o≥3  ⟹  10≤a≤14, 53∤a!−25p=1:\ a!=m^{o}+25,\ a\ge5,\ 5\mid m,\ m\text{ odd},\ o\ge3 \implies 10\le a\le14,\ 5^3\nmid a!-25
Detailed analysis

If p=1p=1, then a!=mo+25a!=m^o+25, and a!≥26a!\ge26 forces a≥5a\ge5, so 5∣m5\mid m. If oo is even then mo≡1(mod4)m^o\equiv1\pmod4 (as mm is odd), giving a!≡2(mod4)a!\equiv2\pmod4, impossible since a≥5a\ge5; hence oo is odd, o≥3o\ge3. Since 53∣mo5^3\mid m^o, we get a!≡25(mod125)a!\equiv25\pmod{125}, which forces 10≤a≤1410\le a\le14; a direct check shows 53∤a!−255^3\nmid a!-25 for each such aa, so p=1p=1 gives no solution.