If the definition of a prime number were changed to call 1 a prime, many statements involving prime numbers would need to be reworded in a more awkward way. For example, the fundamental theorem of arithmetic would need to be rephrased in terms of factorizations into primes greater than 1, because every number would have multiple factorizations with different numbers of copies of 1.  Similarly, the sieve of Eratosthenes would not work correctly if it handled 1 as a prime, because it would eliminate all multiples of 1 (that is, all other numbers) and output only the single number 1.  Some other more technical properties of prime numbers also do not hold for the number 1: for instance, the formulas for Euler's totient function or for the sum of divisors function are different for prime numbers than they are for 1.  By the early 20th century, mathematicians began to agree that 1 should not be listed as prime, but rather in its own special category as a " unit ". 
This autumn, the Albertina Museum in Vienna is paying homage to Raphael with a major presentation of 150 paintings and drawings that has been developed in cooperation with the Ashmolean Museum in Oxford, England. Starting from the Albertina’s own significant holdings and rounded out by the most beautiful and important drawings from prominent museums such as the Uffizi, the Royal Collection of the British Royal Family, the British Museum, the Louvre, the Vatican Museums, and the Ashmolean Museum, this monographic presentation places Raphael’s thinking and mode of conception front and centre: the featured works range from initial spontaneous artist’s impressions to virtuosic detailed studies and compositional studies and on to completed paintings.