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 4 of 5: Solve p=3p=3
p=3: a!=mo+20; a=4⇒(m,o)=(2,2); a=5⇒(m,o)=(10,2); a≥6 impossiblep=3:\ a!=m^{o}+20;\ a=4\Rightarrow(m,o)=(2,2);\ a=5\Rightarrow(m,o)=(10,2);\ a\ge6\text{ impossible}
Detailed analysis

If p=3p=3, then a≥3a\ge3, but a=3a=3 gives a!−26<0a!-26<0, impossible because mo>0m^o>0; hence a≥4a\ge4. Direct computation gives a=4⇒mo=4a=4\Rightarrow m^o=4, so (m,o)=(2,2)(m,o)=(2,2); and a=5⇒mo=100a=5\Rightarrow m^o=100, so (m,o)=(10,2)(m,o)=(10,2). For a≥6a\ge6, 16∣a!16\mid a!, so mo≡−20≡12(mod16)m^o\equiv-20\equiv12\pmod{16}, forcing mm even; if 4∣m4\mid m or o≥3o\ge3 then 8∣mo8\mid m^o, contradicting mo≡4(mod8)m^o\equiv4\pmod8; and if m≡2(mod4)m\equiv2\pmod4 with o=2o=2, then m2≡4(mod16)m^2\equiv4\pmod{16}, so m2+20≡8(mod16)≠0≡a!(mod16)m^2+20\equiv8\pmod{16}\ne0\equiv a!\pmod{16}. Thus a≥6a\ge6 is impossible, leaving exactly the two solutions above.