Another lazy solution

p87.lisp

@@ -0,0 +1,33 @@
+(defun primep (n)
+  (loop for i from 2 upto (sqrt n)
+	do (when (= (mod n i) 0)
+	     (return nil))
+	finally (return t)))
+
+(defun make-prime-enumerator ()
+  (let ((max 1))
+    (lambda ()
+      (loop for n upfrom (1+ max)
+	    until (primep n)
+	    finally (setf max n)
+		    (return n)))))
+
+(defun prime-power-triples (&optional (below 50000000))
+  (let ((solutions (make-hash-table)))
+    (loop with next-prime-a = (make-prime-enumerator)
+	  for a = (funcall next-prime-a)
+	  for square = (* a a)
+	  until (>= square below)
+	  do (loop with next-prime-b = (make-prime-enumerator)
+		   for b = (funcall next-prime-b)
+		   for cube = (* b b b)
+		   for sum-b = (+ square cube)
+		   until (>= sum-b below)
+		   do (loop with next-prime-c = (make-prime-enumerator)
+			    for c = (funcall next-prime-c)
+			    for fourth = (* c c c c)
+			    for sum = (+ square cube fourth)
+			    until (>= sum below)
+			    do (setf (gethash sum solutions) t))))
+    (loop for h being the hash-keys of solutions
+	  sum 1)))