Thank you, Greg and David.

My program shows that 15793939750*R(13860)/R(10)+1 is prime,

however I have little confidence in this result:-)

I hope that the result is as same as David's.

The coefficients of cubic equation t3X^3 + t2X^2 + t1X + t0 = 0 are

t3 = 9499179946...9373462290 1400 digits

t2 = 2983442091...4520303837 2765 digits

t1 = -5786178299...9234821312 4165 digits

t0 = -8297037200...6079883289 2764 digits

The equation has three real roots r1,r2,r3.

r1=-1.433940810...*10^{-1401}

integral part of r2 246804372...4038770926 ( 1382 digits)

fractional part of r2 0.9565089129....

integral part of r3 -246804372...8946383526 ( 1382 digits)

fractional part of r2 0.5941852755....

Nearest integers of r1,r2 and r3 are not root of the equation.

# It was not easy to compute roots of the cubic equation.

Best regards.

Satoshi Tomabechi