Taken at face value, it is nothing more than a postcard of a religious painting, sent by a holidaymaker to one of his closest friends. The signature on the back, however, suggests that all might not be what it seems. Sent by the legendary cryptologist Alan Turing to his psychologist Dr Franz Greenbaum, it appears to be a coded reference to one of history’s most enduring mathematical puzzles, as well as a possible hint to his thoughts of suicide.
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