\[ \boxed { {x}^{11/2}{\frac {{\rm d}^{11}}{{\rm d}{x}^{11}}}y \left ( x \right ) -ay \left ( x \right ) =0} \]
Mathematica: cpu = 0.040505 (sec), leaf count = 670 \[ \left \{\left \{y(x)\to \frac {4}{121} (-1)^{2/11} a^{2/11} c_2 x \, _0F_{10}\left (;-\frac {7}{11},-\frac {5}{11},-\frac {3}{11},-\frac {1}{11},\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11};\frac {2048 a x^{11/2}}{285311670611}\right )-\frac {1048576 (-1)^{9/11} a^{20/11} c_{11} x^{10} \, _0F_{10}\left (;\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11},\frac {23}{11},\frac {25}{11},\frac {27}{11},\frac {29}{11},\frac {31}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{672749994932560009201}-\frac {262144 (-1)^{7/11} a^{18/11} c_{10} x^9 \, _0F_{10}\left (;\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11},\frac {23}{11},\frac {25}{11},\frac {27}{11},\frac {29}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{5559917313492231481}-\frac {65536 (-1)^{5/11} a^{16/11} c_9 x^8 \, _0F_{10}\left (;\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11},\frac {23}{11},\frac {25}{11},\frac {27}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{45949729863572161}-\frac {16384 (-1)^{3/11} a^{14/11} c_8 x^7 \, _0F_{10}\left (;\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11},\frac {23}{11},\frac {25}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{379749833583241}-\frac {4096 \sqrt [11]{-1} a^{12/11} c_7 x^6 \, _0F_{10}\left (;\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11},\frac {23}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{3138428376721}+\frac {1024 (-1)^{10/11} a^{10/11} c_6 x^5 \, _0F_{10}\left (;\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11},\frac {21}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{25937424601}+\frac {256 (-1)^{8/11} a^{8/11} c_5 x^4 \, _0F_{10}\left (;-\frac {1}{11},\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11},\frac {19}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{214358881}+\frac {64 (-1)^{6/11} a^{6/11} c_4 x^3 \, _0F_{10}\left (;-\frac {3}{11},-\frac {1}{11},\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11},\frac {17}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{1771561}+\frac {16 (-1)^{4/11} a^{4/11} c_3 x^2 \, _0F_{10}\left (;-\frac {5}{11},-\frac {3}{11},-\frac {1}{11},\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11},\frac {13}{11},\frac {15}{11};\frac {2048 a x^{11/2}}{285311670611}\right )}{14641}+c_1 \, _0F_{10}\left (;-\frac {9}{11},-\frac {7}{11},-\frac {5}{11},-\frac {3}{11},-\frac {1}{11},\frac {1}{11},\frac {3}{11},\frac {5}{11},\frac {7}{11},\frac {9}{11};\frac {2048 a x^{11/2}}{285311670611}\right )\right \}\right \} \]
Maple: cpu = 6.380 (sec), leaf count = 27500 \[ \text {Too large to display} \]